Implement in-memory DB to run Yorkie without MongoDB

[hackerwins]

What this PR does / why we need it:

If mongo-connection-uri flag is set when the Agent is running, MongoDB is used, otherwise MemDB is used.
When using MemDB, Agent stores its data in memory, so stopping the server will lose all data.

Run Yorkie without MongoDB:

$ ./bin/yorkie agent
2021-11-28T00:24:13.931+0900	INFO	backend created: id: c6h4r7d94810ghl74dhg, rpc: localhost:11101: db: memory
2021-11-28T00:24:13.932+0900	INFO	serving profiling on 11102
2021-11-28T00:24:13.932+0900	INFO	serving RPC on 11101

Run Yorkie with MongoDB:

$ yorkie agent --mongo-connection-uri mongodb://localhost:27017
2021-11-28T00:23:46.842+0900	INFO	MongoDB connected, URI: mongodb://localhost:27017, DB: yorkie-meta
2021-11-28T00:23:46.842+0900	INFO	backend created: id: c6h4r0l94810ggn6obgg, rpc: localhost:11101: db: mongodb://localhost:27017
2021-11-28T00:23:46.843+0900	INFO	serving profiling on 11102
2021-11-28T00:23:46.843+0900	INFO	serving RPC on 11101

• I used github.com/hashicorp/go-memdb for implementation.
• Hexagonal, which I worked with @dc7303 last winter, works well this time as well.

Which issue(s) this PR fixes:

Fixes #275

Special notes for your reviewer:

This PR starts the Memory DB implementation. If we can't finish all in this PR, we can just merge this and wait for help from other contributors.

Tasks:

• ActivateClient
• DeactivateClient
• FindClientInfoByID
• UpdateClientInfoAfterPushPull
• FindDocInfoByKey
• StoreChangeInfos
• FindChangesBetweenServerSeqs
• FindChangeInfosBetweenServerSeqs
• CreateSnapshotInfo
• FindLastSnapshotInfo
• UpdateAndFindMinSyncedTicket

Finally, implemented all DB interfaces in the memory version.

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #276 (d8f7a9d) into main (e544cdb) will decrease coverage by 0.27%.
The diff coverage is 37.46%.

@@            Coverage Diff             @@
##             main     #276      +/-   ##
==========================================
- Coverage   50.21%   49.94%   -0.28%     
==========================================
  Files          53       54       +1     
  Lines        4375     4655     +280     
==========================================
+ Hits         2197     2325     +128     
- Misses       1966     2087     +121     
- Partials      212      243      +31     

Impacted Files	Coverage Δ
pkg/document/document.go	50.72% <0.00%> (-0.75%)	⬇️
yorkie/backend/backend.go	0.00% <0.00%> (ø)
yorkie/backend/db/change_info.go	26.92% <ø> (ø)
yorkie/backend/db/client_info.go	39.62% <0.00%> (-14.23%)	⬇️
yorkie/backend/db/doc_info.go	0.00% <0.00%> (ø)
yorkie/backend/db/mongo/client.go	0.00% <0.00%> (ø)
yorkie/backend/db/mongo/indexes.go	0.00% <0.00%> (ø)
yorkie/backend/db/memory/db.go	48.51% <48.51%> (ø)
yorkie/backend/db/db.go	66.66% <50.00%> (ø)
yorkie/rpc/yorkie_server.go	42.85% <50.00%> (-0.23%)	⬇️
... and 1 more

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[hackerwins]

I used MemoryDB for Rust SDK integration test. Response times were shorter than when using MongoDB.

$ yorkie agent --mongo-connection-uri mongodb://localhost:27017
2021-11-28T21:11:02.915+0900	INFO	MongoDB connected, URI: mongodb://localhost:27017, DB: yorkie-meta
2021-11-28T21:11:02.915+0900	INFO	backend created: id: c6hn3lnblarmnt1sq8f0, rpc: localhost:11101: db: mongodb://localhost:27017
2021-11-28T21:11:02.915+0900	INFO	serving profiling on 11102
2021-11-28T21:11:02.915+0900	INFO	serving RPC on 11101
2021-11-28T21:11:11.959+0900	INFO	RPC : "/api.Yorkie/ActivateClient" 5.954118ms
2021-11-28T21:11:11.962+0900	INFO	RPC : "/api.Yorkie/ActivateClient" 8.416765ms
2021-11-28T21:11:11.963+0900	INFO	RPC : "/api.Yorkie/ActivateClient" 10.306341ms
2021-11-28T21:11:11.966+0900	INFO	RPC : "/api.Yorkie/DeactivateClient" 2.281372ms
^C2021-11-28T21:11:17.952+0900	INFO	Caught signal: interrupt
2021-11-28T21:11:17.956+0900	INFO	backend stoped: id: c6hn3lnblarmnt1sq8f0, rpc: localhost:11101
$ yorkie agent
2021-11-28T21:11:22.569+0900	INFO	backend created: id: c6hn3qnblarmo8ro91d0, rpc: localhost:11101: db: memory
2021-11-28T21:11:22.570+0900	INFO	serving profiling on 11102
2021-11-28T21:11:22.570+0900	INFO	serving RPC on 11101
2021-11-28T21:11:24.439+0900	INFO	RPC : "/api.Yorkie/ActivateClient" 32.881µs
2021-11-28T21:11:24.439+0900	INFO	RPC : "/api.Yorkie/ActivateClient" 91.922µs
2021-11-28T21:11:24.440+0900	INFO	RPC : "/api.Yorkie/ActivateClient" 145.309µs
2021-11-28T21:11:24.442+0900	INFO	RPC : "/api.Yorkie/DeactivateClient" 23.304µs

[dc7303]

Thanks for sharing your awesome work! 👍
I have some questions and suggestions. Please check. 🙇