Fix allgather op_id reuse better

[wence-]

We can't stratify metadata tags by finish or normal metadata because in that scenario we can still accidentally end up posting metadata receives that will be matched by the next collective.

Instead change the event loop to satisfy the correct invariant for metadata receive posting: we post at most one receive per event loop iteration and act on it appropriately. If it is a finish chunk we update the number of expected messages (and hence the number of additional metadata receives we expect to send), otherwise it's a data chunk and we update the number of received messages. This way the metadata receive is never a greedy loop that can eat sends from a subsequent collective.