Massive cleanup of session behavior.

* Move session/token handling to the gateway adapter parent class
* Take numAttempt out of all the forms, because keeping it there is all
kinds of dumb.
* Make a new place for numAttempt in the session.
* Go back to starting people over after three subsequent attempts on one
edit token.
* Handle sessions at the setTransactionWMFStatus level
* Make sure that $_SESSION isn't relied on for anything after
setTransactionWMFStatus is called
* Save fresh donor data to the session, at the start of every
do_transaction instead of... wherever.
* Saving now wipes out old data that is no longer represented
* Resetting session data for subsequent tries no longer murders all
session data. We need some of that to persist indefinitely (velocity,
form stack).
* Rename set/get Transaction WMF Status functions for clarity/gravity.

Change-Id: I4a91e71cf1c84117ff9544dfcd53d2c8864e3ae6