test: complete impl. of msg_merkleblock and wait_for_merkleblock

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.

test/functional/test_framework/messages.py

@@ -1321,10 +1321,23 @@ class msg_headers:
 
 
 class msg_merkleblock:
+    __slots__ = ("merkleblock",)
     command = b"merkleblock"
 
+    def __init__(self, merkleblock=None):
+        if merkleblock is None:
+            self.merkleblock = CMerkleBlock()
+        else:
+            self.merkleblock = merkleblock
+
     def deserialize(self, f):
-        pass  # Placeholder for now
+        self.merkleblock.deserialize(f)
+
+    def serialize(self):
+        return self.merkleblock.serialize()
+
+    def __repr__(self):
+        return "msg_merkleblock(merkleblock=%s)" % (repr(self.merkleblock))
 
 
 class msg_filterload: