Fleshed out the ClientSession state machine to follow the session state graph described by the IMAP specification. ClientSession is now firmly a single-connection object that tracks the server state appropriately. A higher-level class will need to be defined that manages multiple sessions (for when dealing with multiple folders, monitoring, etc.)
The input mode for Deserializer meant that the caller needed to manage the input stream and push data in an appropriate way; this is error-prone. Now Deserializer manages the input stream and the modes it must be read from. Serializer still works in a similar fashion as before, but now it deals with literal data more efficiently, writing it to the output stream directly (via splice_async()) rather than into the in-memory temporary buffer. Serializable's serialize() method is now async, meaning that all serialization can occur asynchronously, which is how we want it going forward.
