The guards are based on spos, which does not change across the call to
_merge_collection, which means we can't open and close multiple guards
within this call. Instead, use a single set of set_guards and closes
surrounding both the collection and temp.
Also, remove the conditional guard around _remove_collection, which we
want to (try to) do (again) as long as we didn't complete our global
replay guard.
Fixes: http://tracker.ceph.com/issues/36610 Signed-off-by: Sage Weil <sage@redhat.com>