BUG-8538: rework transaction abort paths

BUG-8538: rework transaction abort paths

Direct transaction abort path can end up touching proxy history's
maps, which it should not, as that happens only after purge. This
inconsistency has cropped up when purge was introduced.

Refactor the methods so that cohorts are removed only after purge,
and fix abort request routing such that it always enqueues a purge
request (possibly via successor). This also addresses a FIXME, as
we now have an enqueueAbort() request, which is not waiting on the
queue.

Change-Id: Ie291da70ace772274f33505db376a915b38e37c0
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>