WL#1478: Split Session connection and Transaction

This is long term task  targeting heavy enterprise MySQL usage.
In this case database may need to handle very large number of concurrent
sessions (tens of thousands). Having so many threads and even TCP/IP connections
might not
be feasible, if however one connection could handle several sessions transaction 
monitor could do multiplexing, handling several sessions via single connections.
It also allows to save resources - stack resources, connection buffers etc.


It might be also good idea to allow couple of connection to share the same session
(or the same transaction) this would allow to perform large transactions in the
parallel manually, until MySQL is not able to paralellize query automatically.