BUG-8941: enqueue purges once ask-based transactions resolve

Backend state tracking relies on the transaction log to propagate
transaction state from the leader to followers. This includes purging
of transactions, i.e. the information that the frontend will not need
the state (and the final resolution of the transaction).

Tell-based protocol handles this on the frontend, ask-based needs to
do this on the backend (as it has no notion of transaction continuation).

Change-Id: I49e787b38998ef67b4a9ef504a70822263e1a340
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>