gdb: make remote_state's async token private

Make remote_async_inferior_event_token private (rename to
m_async_event_handler_token) and add methods for the various operations
we do on it.  This will help by:

 - allowing to break on those methods when debugging
 - allowing to add assertions in the methods

Change-Id: Ia3b8a2bc48ad4849dbbe83442c3f83920f03334d
Approved-By: Andrew Burgess <aburgess@redhat.com>