Use SRWLock for non-recursive locks on vista and later

After WinXP, Windows added the SRWLock type, which is generally much faster than CRITICAL_SECTION, and supports RW semantics. It doesn't support recursive usage, though it can be hacked in.

I'm calling this 0.2.??? since we have no evidence that locking is anywhere close to our critical path. But who knows; perhaps it is? Or will be, once we do more things in workers?