Add BufferedWriteSyncer

[moisesvega]

This diff addresses comments in uber-go#782

• Add BufferedWriteSyncer
• Use new Clock Interface for testing

API for BufferedWriteSyncer:

type BufferedWriteSyncer struct {
	WriteSyncer
	sync.Mutex

	Size          int
	FlushInterval time.Duration
	Clock         Clock

	// unexported fields for state
}

buf := &bytes.Buffer{}
ws := &BufferedWriteSyncer{WriteSyncer: AddSync(buf)} //  BufferedWriteSyncer with the default values
ws.Write([]byte("foo"))
ws.Close() // Close closes the buffer, cleans up background goroutines, and flushes remaining, unwritten data.

[abhinav]

I know this is not the actual PR so providing some early feedback here.

[abhinav]

FYI, anything you embed is part of the public API. That's probably not desirable for the mutex, but we can do it for the WriteSyncer if we want to.

https://github.com/uber-go/guide/blob/master/style.md#avoid-embedding-types-in-public-structs

[abhinav]

unused field. can delete.

[abhinav]

If we're trying to make Write thread-safe, the initiallized check should also be thread safe.
The lock should be acquired before we change initialized.

[abhinav]

We should use a default clock implementation if unset rather than if-else on using the clock versus using the system.

So the systemClock should be available in zapcore to be used.

[abhinav]

not necessary if leak_test provides this via TestMain