feat(stream): add support for incremental stream writer

[NamanJain8]

This PR adds support for stream writing incrementally to the DB.
Adds an API: StreamWriter.PrepareIncremental

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This change is 

[manishrjain]

Reviewed 6 of 6 files at r3.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @NamanJain8)

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stream_writer.go, line 98 at r3 (raw file):

	var once sync.Once

	// prepareToDrop will stop all the incomming write and flushes any pending flush tasks.

incoming

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stream_writer.go, line 116 at r3 (raw file):

		if level.NumTables > 0 {
			sw.prevLevel = level.Level
		}

let's handle the case where there's no data.