Optimize MPR memory allocator.
    - Run sweeper in parallel with user-threads
    - Use 100% non-blocking, lock-free algorithms
    - Eliminate previous lock contention when allocating
    - Use inline mprMark()
    - Support various block alignments
    - Add mprAllocFast for optimized string cloning
    - Refactor implementation to clarify MprMem layout
Cleanup