BUG-650: AbstractDOMForwardedTransactionFactory

ImmutableMap is not really what we want to track datastores -- EnumMap
has constant time lookups for everything, plus its use eliminates a
needless builder object allocation.

Change-Id: I0bea628b82d8366bf14532d3bc321067584a84b5
Signed-off-by: Robert Varga <rovarga@cisco.com>
BUG-650: speedup AbstractDOMForwardedTransactionFactory

Another speedup -- as it turns out we do not have to take the full lock
to close down the transaction factory. A volatile write is enough to do
that -- which means a volatile read is enough to check for it having
been closed.

Change-Id: I3488ccccc4d91d34665b3ff6e70e047407be48bb
Signed-off-by: Robert Varga <rovarga@cisco.com>