indication when a request can be moved to done queue, but also for
detecting the current state of memsync request.
This approach has problems, e.g. leaking a request if memsynk ack from
the secondary failed, or racy usage of write_complete, which should be
called only once per write request, but for memsync can be entered by
local_send_thread and ggate_send_thread simultaneously.
So the following approach is implemented instead:
1) Use hio_countdown only for counting components we waiting to
complete, i.e. initially it is always 2 for any replication mode.
2) To distinguish between "memsync ack" and "memsync fin" responses
from the secondary, add and use hio_memsyncacked field.
3) write_complete() in component threads is called only before
releasing hio_countdown (i.e. before the hio may be returned to the
done queue).
4) Add and use hio_writecount refcounter to detect when
write_complete() can be called in memsync case.
Reported by: Pete French petefrench ingresso.co.uk
Tested by: Pete French petefrench ingresso.co.uk
MFC after: 2 weeks
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