[tests] Add P2PDataStore class

P2PDataStore subclasses NodeConnCB. This class keeps a store
of txs and blocks and responds to getdata requests from
the node-under-test.

test/functional/test_framework/messages.py

@@ -44,6 +44,11 @@ NODE_UNSUPPORTED_SERVICE_BIT_5 = (1 << 5)
 NODE_UNSUPPORTED_SERVICE_BIT_7 = (1 << 7)
 NODE_NETWORK_LIMITED = (1 << 10)
 
+MSG_TX = 1
+MSG_BLOCK = 2
+MSG_WITNESS_FLAG = 1 << 30
+MSG_TYPE_MASK = 0xffffffff >> 2
+
 # Serialization/deserialization tools
 def sha256(s):
     return hashlib.new('sha256', s).digest()
@@ -204,8 +209,6 @@ class CAddress():
         return "CAddress(nServices=%i ip=%s port=%i)" % (self.nServices,
                                                          self.ip, self.port)
 
-MSG_WITNESS_FLAG = 1<<30
-
 class CInv():
     typemap = {
         0: "Error",