Implement P-Sig key filtering

[hardbyte]

This service doesn't provide an api for client side filter of p-sig blocks, rather all candidate blocks are uploaded and the server joins by block name.

See the blocklib tutorial

A related issue is the Anonlink Entity Service limits block names to 64 characters, which imposes a fairly low limit on p-sig's bloom filter size if we continue to use an encoding of the bloomfilter as the blocking key. Addressed in #633