BUG-8704: rework seal mechanics to not wait during replay

AbstractProxyTransaction.seal() and most notably internalSeal()
can end up pushing down messages down the connection hence they
can end up slowing down the replay process.

The replay paths end up enqueing subsequent requests anyway, so
rework the structure to split the 'seal only' and 'seal and flush'
codepaths.

Change-Id: Ie75c1ef8aa0d3d5d7ca482d383fd516077ca50b4
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>