common/Finisher: add queue(list<Context*>&)

Signed-off-by: Samuel Just <sam.just@inktank.com>
os/: add async flush_commit() method to Sequencer and implementations

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: add machinery to temporarily buffer outgoing recovery messages

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: block outgoing peering messages until the last interval is durable

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branch 'upstream/master' into wip-7999-master

Conflicts:
	src/os/KeyValueStore.h
Merge pull request #2027 from athanatos/wip-7999-master

Wip 7999 master

Fixes: #7999
Backport: Firefly
Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: Haomai Wang <haomaiwang@gmail.com>