Add support for sending private messages

The new UserDestinationMessageHandler resolves messages with
destinations prefixed with "/user/{username}" and resolves them into a
destination to which the user is currently subscribed by appending the
user session id.

For example a destination such as "/user/john/queue/trade-confirmation"
would resolve "/trade-confirmation/i9oqdfzo" assuming "i9oqdfzo" is the
user's session id.