* opsrecv sends an ack back to bgmon for each packet it receives.

* Rework caching code a bit, and rearrange slightly to deal with acks.

Note that opsrecv does not deal with duplicates yet ... still thinking
about that, but it actually does not matter too much if the info gets
duplicated in the DP; multiple entries with the same data and the same
timestamp *should* be harmless.