Refactor arenas array (fixes deadlock).

Refactor the arenas array, which contains pointers to all extant arenas,
such that it starts out as a sparse array of maximum size, and use
double-checked atomics-based reads as the basis for fast and simple
arena_get().  Additionally, reduce arenas_lock's role such that it only
protects against arena initalization races.  These changes remove the
possibility for arena lookups to trigger locking, which resolves at
least one known (fork-related) deadlock.

This resolves #315.