osd/filestore: Change default filestore_merge_threshold to -1

osd/filestore: Change default filestore_merge_threshold to -1

Performance evaluations of medium to large size Ceph clusters have
demonstrated negligible performance impact from unnecessarily deep
directory hierarchies but significant performance impact from filestore
split and merge activity. Disable merges by default.

Fixes: http://tracker.ceph.com/issues/24686
Signed-off-by: Douglas Fuller <dfuller@redhat.com>
(cherry picked from commit 75f55f2dfc92a89ec9d821e00effad75839008c9)

Conflicts:
	src/common/options.cc (retain LEVEL_ADVANCED in luminous because
            65bec679f250ecc8406ac777f373785a3e110031 is not being backported)