Add Go counterpart of Python transaction package[1,2]. The Go version is
not complete - in particular Transaction.Commit is not yet implemented.
However even in this state it is useful to have transaction around for
read-only transaction cases.

The synchronization logic is more well-thought - in particular there is
no dance around "new transaction", because in Go, contrary to ZODB/py,
ZODB connections will be always opened under already started
transaction. See "Synchronization" section in package documentation for
details on this topic.

[1] http://transaction.readthedocs.org
[2] https://github.com/zopefoundation/transaction