HashIndex: randomize split threshold by a configurable amount

HashIndex: randomize split threshold by a configurable amount

Store a random value up to the filestore_split_rand_factor for each
collection when it is created or apply-layout-settings is run. This
should help distribute the load of splitting directories across a
longer period of time.

Fixes: http://tracker.ceph.com/issues/15835
Signed-off-by: Josh Durgin <jdurgin@redhat.com>
(cherry picked from commit 78d0b278a6853a45eda9aa4c78aa0233e07c1299)

Conflicts:
	src/os/filestore/IndexManager.cc (g_conf instead of cct)