I think this will improve the transaction response messages Kuba
mentioned where it was an empty transaction instead of a nil or not
there... if not it should make it easier to do that anyhow.
also adds a "noSleep" option to the server command to avoid the 5
second sleep we introduced on startup for non-coordinator cluster
nodes. The sleep doesn't seem to be needed in the tests and makes them
much slower.
This all needs to be wired into API/Server/Cluster/Holder etc. but I
think the TransactionManager will be a pretty good building block for
managing transaction state at the coordinator level.