FileStore: add global replay guard for split, collection_rename

In the event of a split or collection rename, we need to ensure that
we don't replay any operations on objects within those collections
prior to that point.  Thus, we mark a global replay guard on the
collection after doing a syncfs and make sure to check that in
_check_replay_guard() for all object operations.

Fixes: #5154
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>