BUG-8898: do not invoke timeouts directly

BUG-8898: do not invoke timeouts directly

Request timeouts are occuring with the connection lock held,
at which point the connection can be at the tail of a successor
chain:

oldestConnection -> olderConnection -> connection

If the callback being invoked attempts to transmit an entry,
we will end up attempting to lock the entire chain. This would not
be a problem except that if there is a concurrent attempt to lock
the entire chain it ends up holding the lock of oldestConnection
and it is waiting for the lock on connection -- which will only be
released once the callback finishes executing, but that in turn
waits for oldestConnection to be unlocked -- a classic AB/BA deadlock.

This patch alleviates the problem by deferring callback execution
via executeInActor, i.e. the timeout will be delivered at as part
of normal message processing.

Change-Id: I237908cf214bcdfd477fe0212d09b207a0c2cdbf
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>