connman is in charge of pushing messages

The changes here are dense and subtle, but hopefully all is more explicit
than before.

• CConnman is now in charge of sending data rather than the nodes themselves. This is necessary because many decisions need to be made with all nodes in mind, and a model that requires the nodes calling up to their manager quickly turns to spaghetti.

• The per-node-serializer (ssSend) has been replaced with a (quasi-)const send-version. Since the send version for serialization can only change once per connection, we now explicitly tag messages with INIT_PROTO_VERSION if they are sent before the handshake. With this done, there's no need to lock for access to nSendVersion.
  
  Also, a new stream is used for each message, so there's no need to lock during the serialization process.

• This takes care of accounting for optimistic sends, so the nOptimisticBytesWritten hack can be removed.

• -dropmessagestest and -fuzzmessagestest have not been preserved, as I suspect they haven't been used in years.