Implement a thread pool
    
    This adds a simple thread pool to gdb.  In the end, this seemed
    preferable to the approach taken in an earlier version of this series;
    namely, starting threads in the parallel-foreach implementation.  This
    approach reduces the overhead of starting new threads, and also lets
    the user control (in a subsequent patch) exactly how many worker
    threads are running.
    
    2019-10-19  Christian Biesinger  <cbiesinger@google.com>
    	    Tom Tromey  <tom@tromey.com>
    
    	* gdbsupport/thread-pool.h: New file.
    	* gdbsupport/thread-pool.c: New file.
    	* Makefile.in (COMMON_SFILES): Add thread-pool.c.
    	(HFILES_NO_SRCDIR): Add thread-pool.h.
    
    Change-Id: I597bb642780cb9d578ca92373d2a638efb44fe52