Request: GET_XTHIN
Request the XTHINBLOCK from the peer that was anoounced via the previous INV message.
The message should start with an inventory message that indicates the XTHINBLOCK requested, followed by the Bloom filter generated from the mempool.
Upon receipt of this request, the peer should loads the attached Bloom filter, creates and responds with XTHINBLOCK.
If the XTHINBLOCK cannot be created without hash collisions, the peer may repond with THINBLOCK or full blocks.