kv/KeyValueDB: return const char* from MergeOperator::name()

kv/KeyValueDB: return const char* from MergeOperator::name()

This avoids a c_str() on a temporary string later down the line, which
avoids a use-after-free.

Fixes: http://tracker.ceph.com/issues/26875
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit e9be5dabf256c18a5575d1394e481e4f21ece97d)

Conflicts:
	src/kv/RocksDBStore.cc : Resolved for Name()