tmf: Introduce a scheduler for event requests

tmf: Introduce a scheduler for event requests

The request scheduler will increase the responsiveness of user actions.
It works with 5 slots with a specific time. It has 4 slots for
foreground
requests and 1 slot for background requests, and it passes through all
the
slots (foreground first and background after).

Example: if we have one foreground and one background request, the
foreground
request will be executed four times more often than the background
request.

Change-Id: I82dc1da60eaad92c47e015e052a69473119d7a43
Signed-off-by: Simon Delisle <simon.delisle@ericsson.com>
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/13882
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>