Instead of passing separate engine, part and item components to script APIs, pass down IScriptInstance instead.

This is to allow the future co-operative script thread terminate feature to detect and act upon termination requests.
This splits the assembly and state loading out from the ScriptInstance() constructor to a separate Load() method
in order to facilititate continued script logic regression testing.
Implement co-operative script termination if termination comes during a script wait event (llSleep(), etc.)

This makes use of EventWaitHandles since various web references indicate that Thread.Interrupt() can also cause runtime instability.
If co-op termination is enabled, then termination sets the wait handle instead of waiting for a timeout before possibly aborting the thread.
This allows the script to cleanly terminate if it's in a llSleep/LL function delay or the next time it enters such a wait without any timeout period.
Co-op termination is not yet testable since checking for termination request within loops that never trigger a wait is not yet implemented.
Merge branch 'master' of ssh://opensimulator.org/var/git/opensim