'Add or remove a symlink to/from the sync-containers directory
according to the broker\'s metadata.
Decide according to the broker x-container-sync-to and
x-container-sync-key whether a symlink needs to be added or
removed.
We mention that if both metadata items do not appear
at all, the container has never been set for sync in reclaim_age
in which case we do nothing. This is important as this method is
called for ALL containers from the container replicator.
Once we realize that we do need to do something, we check if
the container is marked for delete, in which case we want to
remove the symlink
For adding a symlink we notice that both x-container-sync-to and
x-container-sync-key exist and are valid, that is, are not empty.
At this point we know we need to do something, the container
is not marked for delete and the condition to add a symlink
is not met. conclusion need to remove the symlink.
:param broker: An instance of ContainerBroker'

def update_sync_store(self, broker):

    if (('X-Container-Sync-To' not in broker.metadata) and ('X-Container-Sync-Key' not in broker.metadata)):
        return
    if broker.is_deleted():
        self.remove_synced_container(broker)
        return
    sync_to = sync_key = None
    if ('X-Container-Sync-To' in broker.metadata):
        sync_to = broker.metadata['X-Container-Sync-To'][0]
    if ('X-Container-Sync-Key' in broker.metadata):
        sync_key = broker.metadata['X-Container-Sync-Key'][0]
    if (sync_to and sync_key):
        self.add_synced_container(broker)
        return
    self.remove_synced_container(broker)
