BSocketMessenger: Further improvements.

- Messages that expect a reply are now tagged with a unique ID field to
indicate that expectation to the receiving socket messenger.
- The messenger now maintains a map of received reply IDs and their
corresponding messages, along with a message queue of other unsolicited
replies.
- After successfully connecting, the messenger now spawns a thread
whose sole responsibility is receiving and parsing all incoming messages,
and consequently sorting them into the aforementioned data structures based
on the presence of the reply ID. Callers who are awaiting either replies or
other messages are signalled appropriately via a semaphore. This allows
multiplexing of both types of messages on the same socket.

libbnetapi: Add socket messenger class.

- Introduces new network API class BSocketMessenger, allowing one to send
and receive BMessages across a network socket in a BMessenger-like
fashion. Still very much WIP, hence currently not exposed via public headers.
Based partly on previous work by Axel.