transactional state isn't carried over with connection._branch()

[Mike Bayer]

eg. the branched connection doesn't take on __transaction. we should probably fix this.

[Mike Bayer]

• Fixed bug where a "branched" connection, that is the kind you get when you call :meth:.Connection.connect, would not share transaction status with the parent. The architecture of branching has been tweaked a bit so that the branched connection defers to the parent for all transactional status and operations. fixes #3190

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