Fix: if calls Close or Dispose many times it'll throws a NullReferenceException

kill the unsent queue when connection fail; this is far more reasonable (expected), and ensures async works correctly

If the incoming socket isn't vanilla, we need to use async-read rather than socket-poll (the value of .Available is meaningless)

Fix command

TIL: Timer can randomly just *stop working* for minutes at a time; added a packemaker via the SocketManager

Prevent writer getting to the back of the Monitor queue; show EVAL example in transactions documentation

Moved from Socket.Select to P/Invoke; I will have to revisit this for Mono, but: Socket.Select simply allocates too many damned arrays

Improved heartbeat code; introduced dedicated reader (can be pooled via SocketManager); tidied solition directory