mds: avoid overlapping release attempts

mds: avoid overlapping release attempts

If the first release attempt is waiting for the log to flush, we
should avoid sending any RELEASED ack until all releases have
flushed.  That is, only the last release will ack.  Keep a counter
in Capability to do this.

Otherwise, we may close out a capability from under a release
that is flushing, and our seq # will be meaningless later in
_finish_release_cap() when we're trying to decide what to do.