Handle ntp sync in audioservice ready wait

Events (any python threading in general) don't use monotonic time and
can be affected by an ntp sync.

This replaces the normal event with the MonotonicEvent class handling jumps
into the future.

mycroft/audio/audioservice.py

@@ -17,11 +17,12 @@
 import time
 from os import listdir
 from os.path import abspath, dirname, basename, isdir, join
-from threading import Lock, Event
+from threading import Lock
 
 from mycroft.configuration import Configuration
 from mycroft.messagebus.message import Message
 from mycroft.util.log import LOG
+from mycroft.util.monotonic_event import MonotonicEvent
 
 from .services import RemoteAudioBackend
 
@@ -152,7 +153,7 @@ def __init__(self, bus):
         self.play_start_time = 0
         self.volume_is_low = False
 
-        self._loaded = Event()
+        self._loaded = MonotonicEvent()
         bus.once('open', self.load_services_callback)
 
     def load_services_callback(self):
@@ -161,7 +162,6 @@ def load_services_callback(self):
             service and default and registers the event handlers for the
             subsystem.
         """
-
         services = load_services(self.config, self.bus)
         # Sort services so local services are checked first
         local = [s for s in services if not isinstance(s, RemoteAudioBackend)]