even faster server startup

[totaam]

Same as #2341, but better. Re-visiting this for #2813.

Server startup times:

• first message 13:29:45,661 get_enabled_encoders(('rencode', 'bencode', 'yaml')) enabled=['rencode', 'bencode', 'yaml']
• vfb startup (quite fast now?)
• dbus startup is slow - over 1 second:

13:29:46,759 dbus_launch=dbus-launch --sh-syntax --close-stderr, current DBUS_SESSION_BUS_ADDRESS=None`
13:29:47,847 dbus_pid=746644, dbus-env={'DBUS_SESSION_BUS_ADDRESS': 'unix:abstract=/tmp/dbus-N1mLcVLF6d,guid=738caecc86fddc584b70219f5ef2f2db', 'DBUS_SESSION_BUS_PID': '746644', 'DBUS_SESSION_BUS_WINDOWID': '2097153'}

can we thread it? (and wait for it to complete in init_dbus_server?)

• gdk init (not bad):

13:29:47,848 X11 display is ready
13:29:47,919 GDK can access the display

• loading ServerBaseClass (~100ms)
• xsettings (~100ms)
• query_sound (500ms) - thread it
• pillow import: 100ms
• **xdg data: 2.5 seconds! ** - we don't need to wait for this to accept connections from newer clients
• nvenc: a few seconds (but worth it)

[totaam]

See also #2556.

[totaam]

Not going to deal with dbus for now because it's too hard to tell which other components will be needing it.

This will do for 4.1:

• r26796 audio init in parallel
• r26797 xdg menu loading in parallel