Support entry version in Write batch

[jarifibrahim]

This PR adds support for setting different versions for different keys in write batch.
The existing implementation of write batch allows setting only single version for all keys in the write batch.
With this PR, user can do the following

wb := db.NewManagedWriteBatch()
wb.SetEntryAt(e, ts)
wb.Flush()

Also, the existing behavior of txn.Commit() in un-managed (normal) mode is to panic, the new behavior would be to return an error.

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This change is 

[manishrjain]

Approved. Do simplify as discussed. @gja can do the final approval.

Reviewed 8 of 8 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ashish-goswami)

[gja]

One more small commit, but otherwise

Reviewable status: 7 of 8 files reviewed, 1 unresolved discussion (waiting on @ashish-goswami, @jarifibrahim, and @manishrjain)

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txn.go, line 475 at r2 (raw file):

	}

	keepTogether := true

I see this logic duplicated twice. Move to txn.keepTogether() please

[gja]

Small Comment sorry

Reviewable status: 7 of 8 files reviewed, 1 unresolved discussion (waiting on @ashish-goswami, @jarifibrahim, and @manishrjain)

[jarifibrahim]

Reviewable status: 7 of 8 files reviewed, 2 unresolved discussions (waiting on @ashish-goswami, @gja, and @manishrjain)

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txn.go, line 475 at r2 (raw file):

Previously, gja (Tejas Dinkar) wrote…

I see this logic duplicated twice. Move to txn.keepTogether() please

One of the instances of keepTogether sets the version for keys, another one just checks if the version is set.

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txn.go, line 478 at r2 (raw file):

	for _, e := range txn.pendingWrites {
		if e.version == 0 {
			e.version = commitTs

Even if there was a single keepTogether function, we would need to keep this loop to assign commitTs to zero-version entries.

[gja]

Reviewable status: 7 of 8 files reviewed, 2 unresolved discussions (waiting on @ashish-goswami, @jarifibrahim, and @manishrjain)

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txn.go, line 475 at r2 (raw file):

Previously, jarifibrahim (Ibrahim Jarif) wrote…

One of the instances of keepTogether sets the version for keys, another one just checks if the version is set.

Oops, that's my bad.