[Ready] Fix _ObjectReader.seek(0, os.SEEK_END) case

[belltailjp]

It addresses #239

This PR fixes the inconsistent behavior of seek when opening a file on S3.

with pfio.v2.open_url('s3://<bucket>/xxxx', 'rb') as f:
    f.seek(0, os.SEEK_END)    # <------ It should move the cursor to the last byte of the stream, but it didn't

Fixing only the above case is enough to fix the issue reported in #239.
But for better transparency of pfio API with other types of filesystems, I updated the whole seek behavior to work identically to normal file-like objects returned by open (binary mode).

[belltailjp]

/test

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[kuenishi]

/test Thanks for contribution! LGTM

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[belltailjp]

/test

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  [UNAVAILABLE] failed to generate serial numbers
  2021-12-22 23:43:19.234414 database.go:348] transaction failed: API error 5 (datastore_v3: TIMEOUT): The datastore operation timed out, or the data was temporarily unavailable. entity groups: [(app=b~pfn-public-ci, kv, "9d344b6a8b151dacbf2144a2d66da50f4b9798ea4af0a044d8a1b5860d4d5ce2"),(app=b~pfn-public-ci, kv, "7137437ea36da4f5eece25816cb5bb03831ecd5ae3a077777df8ec1f368d95e6"),(app=b~pfn-public-ci, kv, "98c3e26365ea16d561df6af9199372b7e72b823c2ab9b9bb8d37326de9062c92"),(app=b~pfn-public-ci, kv, "ccc3a06c2ee72de1a6393b56961a2f898723b93d914b4363b7b6442f65a53dd6"),(app=b~pfn-public-ci, kv, "0e58d640d95b894bf6b6a01104f2049f818affd335ba0c18d422093c6dbdb2fa"),(app=b~pfn-public-ci, kv, "166e13729a2131747e625476b73d28deac37167f5a309cdd91ca108e1e4f3295"),(app=b~pfn-public-ci, kv, "bfdba543a4ae79ea832decce9af412ec5d366d342789ff9d3b8954d0426df740")]
  2021-12-22 23:43:19.234483 generator.go:156] failed to generate serial numbers
  
  Stack trace:
    github.com/pfnet/flexci/internal/frontend/database.transactionOnce (database.go:348)
    github.com/pfnet/flexci/internal/frontend/database.Transaction (database.go:300)
    github.com/pfnet/flexci/internal/frontend/util/idutil.GenerateIDs (generator.go:93)
    github.com/pfnet/flexci/internal/frontend/util/idutil.GenerateID (generator.go:164)
    github.com/pfnet/flexci/internal/frontend/util/idutil.LazyGenerateID.func1 (generator.go:190)
    github.com/pfnet/flexci/internal/frontend/util/idutil.(*LazyIDGenerator).Do.func1 (generator.go:180)
    sync.(*Once).Do (once.go:44)
    github.com/pfnet/flexci/internal/frontend/util/idutil.(*LazyIDGenerator).Do (generator.go:179)
    github.com/pfnet/flexci/internal/frontend/handler/apihandler.(*setJobHandlerFlow).doCreateAction (set_job.go:211)
    github.com/pfnet/flexci/internal/frontend/handler/apihandler.(*setJobHandlerFlow).doAction.func1.1 (set_job.go:173)
    github.com/pfnet/flexci/internal/frontend/handler/apihandler.(*setJobHandlerFlow).doAction.func1 (set_job.go:184)
    github.com/pfnet/flexci/internal/frontend/database.transactionOnce.func1 (database.go:338)
    google.golang.org/appengine/internal.RunTransactionOnce (transaction.go:93)
    google.golang.org/appengine/datastore.RunInTransaction (transaction.go:72)
    github.com/pfnet/flexci/internal/frontend/database.transactionOnce (database.go:336)
    github.com/pfnet/flexci/internal/frontend/database.Transaction (database.go:300)
    github.com/pfnet/flexci/internal/frontend/handler/apihandler.(*setJobHandlerFlow).doAction (set_job.go:169)
    github.com/pfnet/flexci/internal/frontend/handler/apihandler.(*setJobHandlerFlow).Do (set_job.go:57)
    github.com/pfnet/flexci/internal/frontend/handler/apihandler.setJobHandler (set_job.go:31)
    github.com/pfnet/flexci/internal/frontend/handler/apihandler.(*githubWebhookIssueCommentFlow).triggerTest (github_issue_comment.go:289)
    github.com/pfnet/flexci/internal/frontend/handler/apihandler.(*githubWebhookIssueCommentFlow).Do (github_issue_comment.go:100)
    github.com/pfnet/flexci/internal/frontend/handler/apihandler.githubIssueCommentHandler (github_issue_comment.go:48)
    runtime.call64 (asm_amd64.s:523)
    reflect.Value.call (value.go:447)
    reflect.Value.Call (value.go:308)
    github.com/pfnet/flexci/internal/frontend/core.(*registerHandlerFlow).registerHandler.func1.1 (handler.go:184)
    github.com/pfnet/flexci/internal/frontend/core.(*apiHandlerFlow).callHandler (handler.go:472)
    github.com/pfnet/flexci/internal/frontend/core.(*apiHandlerFlow).doInternal (handler.go:324)
    github.com/pfnet/flexci/internal/frontend/core.(*apiHandlerFlow).Do (handler.go:283)
    github.com/pfnet/flexci/internal/frontend/core.(*registerHandlerFlow).registerHandler.func1 (handler.go:181)
    github.com/pfnet/flexci/internal/frontend/core.(*handlerFlow).Do (handler.go:719)
    github.com/pfnet/flexci/internal/frontend/core.(*registerHandlerFlow).Register.func1.1 (handler.go:120)
    github.com/pfnet/flexci/internal/frontend/util/structuredlog.WithContext (log.go:77)
    github.com/pfnet/flexci/internal/frontend/core.(*registerHandlerFlow).Register.func1 (handler.go:116)
    net/http.HandlerFunc.ServeHTTP (server.go:1964)
    net/http.(*ServeMux).ServeHTTP (server.go:2361)
    github.com/pfnet/flexci/internal/common/api.callInternal.func2 (call.go:204)
    github.com/pfnet/flexci/internal/common/api.callInternal (call.go:212)
    github.com/pfnet/flexci/internal/common/api.Call (call.go:128)
    github.com/pfnet/flexci/internal/common/api.GithubIssueComment (call.go:519)
    github.com/pfnet/flexci/internal/frontend/handler/xternalhandler.(*githubHookFlow).doInternal (github_webhook.go:146)
    github.com/pfnet/flexci/internal/frontend/handler/xternalhandler.(*githubHookFlow).Do (github_webhook.go:39)
    github.com/pfnet/flexci/internal/frontend/handler/xternalhandler.githubWebhookHandler (github_webhook.go:29)
    github.com/pfnet/flexci/internal/frontend/core.(*handlerFlow).Do (handler.go:719)
    github.com/pfnet/flexci/internal/frontend/core.(*registerHandlerFlow).Register.func1.1 (handler.go:120)
    github.com/pfnet/flexci/internal/frontend/util/structuredlog.WithContext (log.go:81)
    github.com/pfnet/flexci/internal/frontend/core.(*registerHandlerFlow).Register.func1 (handler.go:116)
    net/http.HandlerFunc.ServeHTTP (server.go:1964)
    net/http.(*ServeMux).ServeHTTP (server.go:2361)
    google.golang.org/appengine/internal.executeRequestSafely (api.go:165)
    google.golang.org/appengine/internal.handleHTTP (api.go:124)
    net/http.HandlerFunc.ServeHTTP (server.go:1964)
    net/http.serverHandler.ServeHTTP (server.go:2741)
    net/http.(*conn).serve (server.go:1847)
    runtime.goexit (asm_amd64.s:1333)

[belltailjp]

/test

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