BUG-8538: do not invoke read callbacks during replay.

BUG-8538: do not invoke read callbacks during replay.

As evidenced by a ConcurrentModificationException happening reliably
in face of aborted read-only transactions, there are avenues how
our state can be modified eventhough we hold the locks.

One such avenue is listeners hanging on read operations, which
can enqueue further requests in the context of calling thread. That
thread must not be performing replay, hence delay request completion
into a separate actor message by using executeInActor().

Change-Id: Ibcd0ac788156011ec3a4cc573dc7fb249ebf93a2
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>