Implements the missing initialization/serialization methods for
msg_merkleblock, based on the already present class CMerkleBlock.
Also changes the method wait_for_merkleblock() to be more precise by waiting
for a merkleblock with a specified blockhash instead of an arbitrary one.
In the BIP37 test p2p_filter.py, this new method is used to make the test of
receiving merkleblock and tx if a filter is set to be more precise, by checking
if they also arrive in the right order.