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>
(cherry picked from commit
1e07329c0d800b8fea43ae0c4060aded5fd18739)