Change thread_to_thread_object to return a new reference

Change thread_to_thread_object to return a new reference

This changes thread_to_thread_object to return a new reference and
fixes up all the callers.

gdb/ChangeLog
2018-09-16  Tom Tromey  <tom@tromey.com>

	* python/python-internal.h (thread_to_thread_object): Change
	return type.
	* python/py-inferior.c (thread_to_thread_object): Return a new
	reference.
	(infpy_thread_from_thread_handle): Update.
	* python/py-infthread.c (gdbpy_selected_thread): Update.
	* python/py-stopevent.c (create_stop_event_object): Update.
	* python/py-threadevent.c (py_get_event_thread): Return a new
	reference.
	(py_get_event_thread): Update.
	* python/py-event.h (py_get_event_thread): Change return type.
	* python/py-continueevent.c (create_continue_event_object):
	Update.