rgw: radosgw-admin object unlink

Add a radosgw-admin option to remove object from bucket index

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 16235a7acb9543d60470170bb2a09956364626cd)

Conflicts:
	src/rgw/rgw_admin.cc
	src/rgw/rgw_rados.h
	src/test/cli/radosgw-admin/help.t
rgw: a tool to fix buckets with leaked multipart references

Checks specified bucket for the #4011 symptoms, optionally fix
the issue.

sytax:
  radosgw-admin bucket check --bucket=<bucket> [--fix]

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 2d8faf8e5f15e833e6b556b0f3c4ac92e4a4151e)

Conflicts:
	src/rgw/rgw_admin.cc
	src/rgw/rgw_rados.h