message: Move size limits to module-global
  As well, this renames those variables to match PEP8 and this clears up
  the comment relating to VALID_DATA_LIMIT.
  Admittedly, this commit is mainly to make the following ones cleaner.
diff: @@ -23,6 +23,8 @@ from test_framework.mininode import (
  )
  from test_framework.test_framework import BitcoinTestFramework
  +MSG_LIMIT = 4 * 1000 * 1000 # 4MB, per MAX_PROTOCOL_MESSAGE_LENGTH
  +VALID_DATA_LIMIT = MSG_LIMIT - 5 # Account for the 5-byte length prefix
  class msg_unrecognized:
  """Nonsensical message. Modeled after similar types in test_framework.messages."""
  @@ -61,8 +63,6 @@ class InvalidMessagesTest(BitcoinTestFramework):
  node.add_p2p_connection(P2PDataStore())
  conn2 = node.add_p2p_connection(P2PDataStore())
  - msg_limit = 4 * 1000 * 1000 # 4MB, per MAX_PROTOCOL_MESSAGE_LENGTH
  - valid_data_limit = msg_limit - 5 # Account for the 4-byte length prefix
  #
  # 0.
  @@ -70,8 +70,8 @@ class InvalidMessagesTest(BitcoinTestFramework):
  # Send as large a message as is valid, ensure we aren't disconnected but
  # also can't exhaust resources.
  #
  - msg_at_size = msg_unrecognized(str_data="b" * valid_data_limit)
  - assert len(msg_at_size.serialize()) == msg_limit
  + msg_at_size = msg_unrecognized(str_data="b" * VALID_DATA_LIMIT)
  + assert len(msg_at_size.serialize()) == MSG_LIMIT
  self.log.info("Sending a bunch of large, junk messages to test memory exhaustion. May take a bit...")
  