Complete overhaul of the concurrency-related aspects of the library (threading, atomics, locking, etc.), adding private routines in the H5TS package to allow internal algorithms to use all of these capabilities.
Adds many new features & components in the H5TS package that are equivalent to common concurrency data structures and capabilities: "regular" and recursive mutices, condition variables, semaphores, thread barriers, 'once' support, thread pools, atomic variables, thread-local keys, and spawning & joining internal threads.
Now supports C11, pthreads, and Windows threading for all H5TS capabilities, except the recursive readers/writers lock, which is not supported on Windows (because Windows threads don't provide a callback on thread-local variable deletion).
The "global" API lock is switched to use a recursive mutex from the H5TS package, instead of its own variant.
API context code (H5CX package) and error stacks (H5E package) now use the common thread-local info, instead of their own variants.
Subfiling code is switched from using Mercury threading features to the new internal H5TS features.
Removes the mercury threading code.
Adds a configure option (--enable-threads / HDF5_ENABLE_THREADS), enabled by default, to control whether threading is enabled within the library.
