Simplified SelectorManager state machine

Removed changing state, as it is just an aspect of processing.
447627  MultiPart file always created when "filename" set in Content-Disposition
448156 Fixed INACTIVE race in IteratingCallback

Simplified the ICB statemachine with the addition of a lock state to handle the case of iterate() called while a
previous process() iteration is just existing.

Also added the CALLED state to replace INACTIVE when a callback is called before PENDING state entered.

renamed INACTIVE to IDLE
Merge remote-tracking branch 'origin/jetty-9.2.x'