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Writeback Throttle
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Previously, the filestore had a problem when handling large numbers of
small ios.  We throttle dirty data implicitely via the journal, but
a large number of inodes can be dirtied without filling the journal
resulting in a very long sync time when the sync finally does happen.
The flusher was not an adequate solution to this problem since it
forced writeback of small writes too eagerly killing performance.

WBThrottle tracks unflushed io per hobject_t and ::fsyncs in lru
order once the start_flusher threshhold is exceeded for any of
dirty bytes, dirty ios, or dirty inodes.  While any of these exceed
the hard_limit, we block on throttle() in _do_op.

See src/os/WBThrottle.h, src/osd/WBThrottle.cc

To track the open FDs through the writeback process, there is now an
fdcache to cache open fds.  lfn_open now returns a cached FDRef which
implicitely closes the fd once all references have expired.

Filestore syncs have a sideeffect of flushing all outstanding objects
in the wbthrottle.

lfn_unlink clears the cached FDRef and wbthrottle entries for the
unlinked object when then last link is removed and asserts that all
outstanding FDRefs for that object are dead.