rgw: limit entries in remove_olh_pending_entries()

rgw: limit entries in remove_olh_pending_entries()

If there are too many entries to send in a single osd op, the osd rejects
the request with EINVAL. This error happens in follow_olh(), which means
that requests against the object logical head (requests with no version
id) can't be resolved to the current object version. In multisite, this
also causes data sync to get stuck in retries

Fixes: http://tracker.ceph.com/issues/39118
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 3805ea635a5f28656072b2cf5cdc9b5bf53b5e97)

Conflicts:
	src/rgw/rgw_rados.cc
- luminous ref.ioctx.operate() call takes ref.oid instead of ref.obj.oid