Callback fixes

Incomplete, I need to do more and write more tests, which my tests are currently failing/hanging.

However, this fixes a few games (probably more I don't have) and also includes perf fixes / cleanup:
 * Moves delete thread logic into a function instead of several places, and uses SceUID instead of ptrs.
 * Saves ~1.6% of time by not checking for callbacks on every resched (Win32 release, probably not as good elsewhere.)
 * Fixes running multiple callbacks in a row.
 * Fixes interrupts stalling the game if they happened during a callback.

I've tested this on a lot of my games just now, and it didn't hurt any of them.  It fixes at least Exit, and Hexyz Force.

-[Unknown]