Remove IoFuture from session interfaces.

This means that async handlers that want to use the session will
need to make sure they dispatch to a worker first
Revert "Add option to immediatly dispatch to a worker thread"

This reverts commit 6c3125955decfaa218518f502e07edc58d6a4067.
Adjust the dispatch strategy based on if we are pipelining or not
Remove unused messages
Add optimised output stream for use in blocking handlers
Remove the use of IoFuture from the security API