Extract all locking state into a separate struct that will encapsulate locking and state change behavior.

This struct should add or subsume at least the following:
* alive
* closingLock
* ctxInProgress (though this may be restructured because it's possible a Tx may have a ctx and a query run in that Tx could have one)
* busy
* lock/unlock
* Tx in-progress
* Rows in-progress
* ConnPool checked-out or checked-in - maybe include reference to conn pool