Add maint set/show worker-threads

Add maint set/show worker-threads

This adds maint commands to control the number of worker threads that
gdb can use.

2019-11-26  Tom Tromey  <tom@tromey.com>

	* NEWS: Add entry.
	* maint.c (_initialize_maint_cmds): Add "worker-threads" maint
	commands.  Call update_thread_pool_size.
	(update_thread_pool_size, maintenance_set_worker_threads): New
	functions.
	(n_worker_threads): New global.

gdb/doc/ChangeLog
2019-11-26  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Maintenance Commands): Document new maint
	commands.

Change-Id: I4fb514faa05879d8afe62c77036a4469d57dca2a