Alphabetizes includes and resolves quite a few instances of indirect
inclusions, making the requirements of several interfaces explicit. This
also trims out includes that aren't actually necessary (likely due to
changes in the API over time).
Now that we have the fencing and atomic operations in place to ensure
access to data on other threads will always occur before the use of
delete, we can remove the destructor lock. This will be useful for
making ObjToken's move assignment operator noexcept.