DAP: Allow for deferring stop events from gdb thread

DAP: Allow for deferring stop events from gdb thread

The existing `send_event_later()` method allows commands processed on
the DAP thread to queue an event for execution until after the response
has been sent to the client.

We now introduce a corresponding method for use by the gdb thread. This
method `send_event_maybe_later()` will queue the event just like
`send_event_later()`, but only if it has been configured to do so by a
new @request option `defer_stop_events`. As the name implies the
functionality is currently only used for handling stop events.

Approved-By: Tom Tromey <tom@tromey.com>