Refactor the pooled ArenaAllocator.

Refactor the pooled ArenaAllocator.

The mark/release functionality on `ArenaAllocator` was only being used by the pooled
allocator, and caused some complications in API and implementation. Instead of
providing this functionality on all allocators, refactor this functionality out of
`ArenaAllocator` and into a new subclass, `PooledAllocator`, that provides the
singleton pooled allocator.

A number of additional usability/reliability changes have been enabled as part of this
change:
- `ArenaAllocator::initialize` has been replaced with move assignment
- Asserts have been added on all relevant instance methods to ensure that
  `ArenaAllocator` values are initialized before use
- `ArenaAllocator::returnPooledAllocator` has been replaced by making
  `ArenaAllocator::destroy` virtual and having `PooledAllocator::destroy` return the
  singleton
- The teardown of the pooled allocator is now thread-safe w.r.t. the shutdown of the
  arena allocator subsystem