Expose internal thread states to allow applications to track idleness

There have recently been changes in MongoDB aimed at improving diagnosability and debuggability by tracking some context for threads in the system. The particular information currently tracked is:

• A name for the thread
• Whether the thread is currently idle or active

It would be nice if threads in the WiredTiger library could share the same context with MongoDB. The API in MongoDB can be found here:
https://github.com/mongodb/mongo/blob/master/src/mongo/util/concurrency/idle_thread_block.cpp