FIX: mprDestroy in cases where it cannot complete due to running threads or services.

- Rename mprRequestGC to mprGC (old name deprecated but supported)
- Rename MprHeap.enabled to MprHeap.gcEnabled
- Clarify mprYield and mprGC flag documentation
- Rename MprThread.waitForGC to waitForSweeper
- Remove MPR_YIELD_BLOCK option - complicated mprYield and not required
- Change mprDestroy
	- Return bool if destroy cannot complete due to running threads or services
	- Ensure mprGC(COMPLETE) cannot block inside mprDestroy
- Improve and clarify comments