Allow specifying dbtx in TxnBegin.
Allow specifying dbtx in TxnBegin.

Optionally allow specifying an existing DbTxn to use in TxnBegin of the
DBs, so that one can use an atomic transaction across multiple databases
to fix syncing issues in the future.

fsb4000/huntercoin,wiggi/huntercoin,fsb4000/huntercoin,domob1812/huntercoin,domob1812/huntercoin,fsb4000/huntercoin,wiggi/huntercoin,chronokings/huntercoin,chronokings/huntercoin,domob1812/huntercoin,fsb4000/huntercoin,chronokings/huntercoin,chronokings/huntercoin,wiggi/huntercoin,domob1812/huntercoin,wiggi/huntercoin