#903: Ensure transaction ID is passed through to connection during commit.

[tseaver]

Fixes #903.

[dhermes]

LGTM. Seems leakier than need be (i.e. we could put the ID in the commit signature and Batch wouldn't need to know about the ID)

Pretty scary that was happening

[tseaver]

Having transaction_id in the signature of Transaction.commit would be pretty silly.

My initial pass gave Batch.commit() an optional _transaction_id parameter, documented as "for use only by Transaction.commit(), but pylint put the kibosh on it (it doesn't want the subclass to have a different signature than the superclass.

[coveralls]

Changes Unknown when pulling 164e1d8 on tseaver:903-pass_xact_id_during_commit into * on GoogleCloudPlatform:master*.