os/FileJournal: stop aio completion thread *after* writer thread

The writer thread may submit a new aio to update the header in its
final moments before shutting down.  Do not stop the aio thread until after
that has happened or else we may not wait for those aio completions.

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit c776a89880fdac270e6334ad8e49fa616d05d0d4)

Conflicts:
	src/os/FileJournal.cc
Merge pull request #2724 from dachary/wip-9073-journal-aio-mode-firefly

os/FileJournal: stop aio completion thread *after* writer thread