Use exclusive RNG lock and replace old RNG

This commit does two things:

1. It allows `withSeed` to call the given function `fun` with a new
rng (which seed has been appropriately set), but at the same time
restores the previous RNG once we finish running the action;

2. It guards the inner action via a mutex so that we can get thread
   safety. This is implemented via `withGlobalRNGLock`.