GetMappedRange is safe to call anytime

[kainino0x]

I think we can guarantee this.

From #481 (comment):

@lokokung:
Do you think "always unsafe" would be too pessimistic? I'm wondering if we could say "potentially unsafe"? (Mainly I'm thinking about getMappedRange calls in MapAsync callback which would be nice to use in spontaneous mode)

@kainino0x:
I prefer "always unsafe" to avoid tempting too much questioning of when it's safe. That said, am I remembering correctly that it's never really unsafe, it's just that it can cause a self-deadlock if it tries to take a lock that's already held on the same thread?

Also, for GetMappedRange maybe we could specifically document that it's safe to call in any callback (or at least in BufferMap and OnSubmittedWorkDone). That would require the implementation to guarantee either of the following:

• said callbacks won't called spontaneously while holding any lock that GetMappedRange needs
• GetMappedRange doesn't take any locks (it probably doesn't need them, it just needs an atomic [wrapped around the nullable mapping base pointer])