Simplify OpCtx::markKilled

Operation context currently relies on an elaborate dance between the client lock, _waitMutex, _waitCV and _numKillers to allow markKilled to tap the condvar an opctx is waiting on.

After the introduction of batons on all opctx's, this is no longer necessary (as batons have their own support for being woken, while waiting on a condvar). Removing the special killing code will simplify opctx, and remove a lot of extra book keeping.