gthreadpool: set default max_unused_threads and max_idle_time values

gthreadpool: set default max_unused_threads and max_idle_time values

GThreadPool defaulted to 0 for max_unused_threads (meaning thread-pool
threads would exit immediately if there was not already another task
waiting for them), and 0 for max_idle_time (meaning unused threads
would linger forever, though this is only relevant if you changed
max_unused_threads).

However, GIOScheduler changed the global defaults to 2 and 15*1000,
respectively, arguing that these were more useful defaults. And they
are, so let's use them.

https://bugzilla.gnome.org/show_bug.cgi?id=661767