Replace inf_threads_iterator with next_iterator

This changes inf_threads_iterator and some range adapters in
thread-iter.h to use next_iterator and next_adapter instead.

gdb/ChangeLog
2019-01-10  Tom Tromey  <tom@tromey.com>

	* thread-iter.h (inf_threads_iterator): Use next_iterator.
	(basic_inf_threads_range): Remove.
	(inf_threads_range, inf_non_exited_threads_range)
	(safe_inf_threads_range): Use next_adapter.