Support WriteBatch API in managed mode

[jarifibrahim]

This PR adds a new WriteBatchAt(commitTs) API which allows user to use write batch in managed mode.

Fixes #944

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This change is 

[ashish-goswami]

Reviewable status: 0 of 6 files reviewed, 1 unresolved discussion (waiting on @ashish-goswami, @jarifibrahim, and @manishrjain)

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batch.go, line 51 at r1 (raw file):

// NewWriteBatchAt is similar to NewWriteBatch but it allows user to set the commit timestamp.
// NewWriteBatchAt is supposed to be used in the managed mode.
func (db *DB) NewWriteBatchAt(commitTs uint64) (*WriteBatch, error) {

should we move function to managed_db.go?

[manishrjain]

Don't return error. And avoid making other changes.

Reviewed 6 of 6 files at r1.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @jarifibrahim)

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batch.go, line 44 at r1 (raw file):

	if db.opt.managedTxns {
		return nil,
			errors.New("cannot use NewWriteBatch in managed mode. Use NewWriteBatchAt instead")

Make it panic. No need for error. So, don't change any existing APIs.

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batch.go, line 54 at r1 (raw file):

	if !db.opt.managedTxns {
		return nil, errors.New(
			"cannot use NewWriteBatchAt with managedDB=false. Use NewTransaction instead")

Make this panic as well.

[jarifibrahim]

Reviewable status: 0 of 3 files reviewed, 3 unresolved discussions (waiting on @ashish-goswami and @manishrjain)

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batch.go, line 44 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Make it panic. No need for error. So, don't change any existing APIs.

Done.

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batch.go, line 51 at r1 (raw file):

Previously, ashish-goswami (Ashish Goswami) wrote…

should we move function to managed_db.go?

Moved to managed_db.go

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batch.go, line 54 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Make this panic as well.

Done.