librbd: improve object map performance under high IOPS workloads

librbd: improve object map performance under high IOPS workloads

Do not zero-fill the BitVector's bitset prior to decoding the data.
Additionally, only read-update-modify the portions of the footer
that are potentially affected by the updated state.

Fixes: http://tracker.ceph.com/issues/38538
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 071671fff64f27943047610fe075a7e98f0f705c)

Conflicts:
	src/cls/rbd/cls_rbd.cc
	src/cls/rbd/cls_rbd_client.h
	src/common/bit_vector.hpp
	src/test/common/test_bit_vector.cc
	src/test/librbd/test_ObjectMap.cc
Trivial conflicts with bufferlist::begin/cbegin, assert/ceph_assert,
and encode/decode.