librbd: batch ObjectMap updations upon trim

librbd: batch ObjectMap updations upon trim

Shrinking a clone which has snapshots and does not share
majority of objects with its parent (i.e., there are less
objects to be copied up) involves huge number of object
map updates -- two (pre, post) per object. This results
in lots of requests to be send to OSDs especially when
trimming a gigantus image. This situation can be optimized
by sending batch ObjectMap updates for an object range
thereby significantly cutting down OSD traffic resulting
in faster trim times.

Fixes: http://tracker.ceph.com/issues/17356
Signed-off-by: Venky Shankar <vshankar@redhat.com>