Replace setInventoryKnown with a rolling bloom filter.

Mruset setInventoryKnown was reduced to a remarkably small 1000
entries as a side effect of sendbuffer size reductions in 2012.

This removes setInventoryKnown filtering from merkleBlock responses
because false positives there are especially unattractive and
also because I'm not sure if there aren't race conditions around
the relay pool that would cause some transactions there to
be suppressed. (Also, ProcessGetData was accessing
setInventoryKnown without taking the required lock.)