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.
The interfaces for the methods wait_for_header() and wait_for_merkleblock() are
changed to take a hex string instead of an integer, improving type safety and
removing the burden from the caller to always do the transformation via
int(...). As suggested by MarcoFalke in
https://github.com/bitcoin/bitcoin/pull/18593#discussion_r407062253
This is a backport of Bitcoin Core PR18593