Decouple connector from Server

Before we move on to upgrading messaging server to using NIO, it make sense to decouple the part of functionality that deals with I/O into separate component that can be easily swapped. Therefore I have introduced Connector. Communication between connector and server is done via two queues: one queue is input queue for the server and output queue for the connector and another one - output queue for the server and input queue for the connector. These queues are holding SessionCommand objects. At the moment, two types of SessionCommands are supported:
1) QUEUE (connector->server). When a new client connection is made, connector creates a queue that will pass data frames from the server via connector to the client. This queue is specified in the QUEUE command that connector sends to the server. This command also included unique session ID, generated by the connector.
2) QUEUE (server->connector). Once the server has received the QUEUE command from the connector, it creates another queue (or re-uses previously created one) that will pass data frames from the client via connector to the server. This queue is specified in the QUEUE command that the server sends back to the connector.
3) CLOSE (connector->server). If client closes its connection, or an I/O exception happens during reading/writing on the client connection, connector sends CLOSE command to the server, informing that session has been closed. If the cause was an exception, it will be passed in the command.
4) CLOSE (server->connector). Server may decide to close session, so it asks connector to close connection by sending CLOSE command. At the moment, this use case is not present in the code.

Decoupling means that the server now works only with SessionCommands and data frames (class Frame). There are no callbacks between server and connector either, so there is no thread-dependencies.

QueueSizeMonitor is now injected in both server and connector. They update counters for the distribution queue and output queue whenever they put/take values from them.