Fix TestSnapshotV3RestoreMultiMemberAdd flakes (leaks)

[ptabor]

• most important: unix's socket transport should not keep idle
  connections. For top-level Transport we close them using:
  

  etcd/server/etcdserver/api/rafthttp/transport.go

  Line 226 in f3c5180

  tr.CloseIdleConnections()

  
  but currently we don't have access to close them witing the nest (unix) transport. Short idle deadline is good enough.

• Use dialContext (instead of dial) to make sure context is passed down the stack

• Make sure Context is cancelled as soon as the operation is done in pipeline

• nit: use dedicated method to yeld goroutines.

---

Tested with:

d=$(date +"%Y%m%d_%H%M")
(cd tests && go test --timeout=60m ./integration/snapshot -run TestSnapshotV3RestoreMultiMemberAdd -v --count=180 2>&1 | tee log_${d}.log)

---

There were transports & cmux leaked:

   leak.go:118: Test appears to have leaked a Transport:
        internal/poll.runtime_pollWait(0x7f6c5c3784c8, 0x72, 0xffffffffffffffff)
        	/usr/lib/google-golang/src/runtime/netpoll.go:222 +0x55
        internal/poll.(*pollDesc).wait(0xc003296298, 0x72, 0x0, 0x18, 0xffffffffffffffff)
        	/usr/lib/google-golang/src/internal/poll/fd_poll_runtime.go:87 +0x45
        internal/poll.(*pollDesc).waitRead(...)
        	/usr/lib/google-golang/src/internal/poll/fd_poll_runtime.go:92
        internal/poll.(*FD).Read(0xc003296280, 0xc0031f60a8, 0x18, 0x18, 0x0, 0x0, 0x0)
        	/usr/lib/google-golang/src/internal/poll/fd_unix.go:166 +0x1d5
        net.(*netFD).Read(0xc003296280, 0xc0031f60a8, 0x18, 0x18, 0x18, 0xc0009056e2, 0x203000)
        	/usr/lib/google-golang/src/net/fd_posix.go:55 +0x4f
        net.(*conn).Read(0xc000010258, 0xc0031f60a8, 0x18, 0x18, 0x0, 0x0, 0x0)
        	/usr/lib/google-golang/src/net/net.go:183 +0x91
        github.com/soheilhy/cmux.(*bufferedReader).Read(0xc0003d24e0, 0xc0031f60a8, 0x18, 0x18, 0xc0003d24d0, 0xc0009056e2, 0xc000278400)
        	/home/ptab/private/golang/pkg/mod/github.com/soheilhy/cmux@v0.1.5/buffer.go:53 +0x12d
        github.com/soheilhy/cmux.hasHTTP2Preface(0x1367e20, 0xc0003d24e0, 0x7f6c5c699f40)
        	/home/ptab/private/golang/pkg/mod/github.com/soheilhy/cmux@v0.1.5/matchers.go:195 +0x8a
        github.com/soheilhy/cmux.matchersToMatchWriters.func1(0x7f6c5c699f40, 0xc000010258, 0x1367e20, 0xc0003d24e0, 0xc000010258)
        	/home/ptab/private/golang/pkg/mod/github.com/soheilhy/cmux@v0.1.5/cmux.go:128 +0x39
        github.com/soheilhy/cmux.(*cMux).serve(0xc003228690, 0x138c410, 0xc000010258, 0xc00327f740, 0xc0059ba860)
        	/home/ptab/private/golang/pkg/mod/github.com/soheilhy/cmux@v0.1.5/cmux.go:192 +0x1e7
        created by github.com/soheilhy/cmux.(*cMux).Serve
        	/home/ptab/private/golang/pkg/mod/github.com/soheilhy/cmux@v0.1.5/cmux.go:179 +0x191

        internal/poll.runtime_pollWait(0x7f6c5c60f3f0, 0x72, 0xffffffffffffffff)
        	/usr/lib/google-golang/src/runtime/netpoll.go:222 +0x55
        internal/poll.(*pollDesc).wait(0xc000d53018, 0x72, 0x1000, 0x1000, 0xffffffffffffffff)
        	/usr/lib/google-golang/src/internal/poll/fd_poll_runtime.go:87 +0x45
        internal/poll.(*pollDesc).waitRead(...)
        	/usr/lib/google-golang/src/internal/poll/fd_poll_runtime.go:92
        internal/poll.(*FD).Read(0xc000d53000, 0xc000cfd000, 0x1000, 0x1000, 0x0, 0x0, 0x0)
        	/usr/lib/google-golang/src/internal/poll/fd_unix.go:166 +0x1d5
        net.(*netFD).Read(0xc000d53000, 0xc000cfd000, 0x1000, 0x1000, 0x3, 0x3, 0x1000000000001)
        	/usr/lib/google-golang/src/net/fd_posix.go:55 +0x4f
        net.(*conn).Read(0xc00031a570, 0xc000cfd000, 0x1000, 0x1000, 0x0, 0x0, 0x0)
        	/usr/lib/google-golang/src/net/net.go:183 +0x91
        net/http.(*persistConn).Read(0xc00093b320, 0xc000cfd000, 0x1000, 0x1000, 0x577750, 0x60, 0x0)
        	/usr/lib/google-golang/src/net/http/transport.go:1933 +0x77
        bufio.(*Reader).fill(0xc005702fc0)
        	/usr/lib/google-golang/src/bufio/bufio.go:101 +0x108
        bufio.(*Reader).Peek(0xc005702fc0, 0x1, 0xc00077c660, 0xc003b082a0, 0xc000d08de0, 0x5ae586, 0x11dd6c0)
        	/usr/lib/google-golang/src/bufio/bufio.go:139 +0x4f
        net/http.(*persistConn).readLoop(0xc00093b320)
        	/usr/lib/google-golang/src/net/http/transport.go:2094 +0x1a8
        created by net/http.(*Transport).dialConn
        	/usr/lib/google-golang/src/net/http/transport.go:1754 +0xdaa

        net/http.(*persistConn).writeLoop(0xc00093b320)
        	/usr/lib/google-golang/src/net/http/transport.go:2393 +0xf7
        created by net/http.(*Transport).dialConn
        	/usr/lib/google-golang/src/net/http/transport.go:1755 +0xdcf

        sync.runtime_Semacquire(0xc0059ba868)
        	/usr/lib/google-golang/src/runtime/sema.go:56 +0x45
        sync.(*WaitGroup).Wait(0xc0059ba860)
        	/usr/lib/google-golang/src/sync/waitgroup.go:130 +0x65
        github.com/soheilhy/cmux.(*cMux).Serve.func1(0xc003228690, 0xc0059ba860)
        	/home/ptab/private/golang/pkg/mod/github.com/soheilhy/cmux@v0.1.5/cmux.go:158 +0x56
        github.com/soheilhy/cmux.(*cMux).Serve(0xc003228690, 0x13698c0, 0xc00377a0f0)
        	/home/ptab/private/golang/pkg/mod/github.com/soheilhy/cmux@v0.1.5/cmux.go:173 +0x115
        go.etcd.io/etcd/server/v3/embed.(*Etcd).servePeers.func1(0xc0007cc360, 0x122b75f)
        	/home/ptab/corp/etcd/server/embed/etcd.go:518 +0x2b9
        go.etcd.io/etcd/server/v3/embed.(*Etcd).servePeers.func3(0xc00036d080, 0xc0059330a0)
        	/home/ptab/corp/etcd/server/embed/etcd.go:549 +0x182
        created by go.etcd.io/etcd/server/v3/embed.(*Etcd).servePeers
        	/home/ptab/corp/etcd/server/embed/etcd.go:543 +0x73a
--- FAIL: TestSnapshotV3RestoreMultiMemberAdd (17.74s)

[ptabor]

Thank you for review !