sensor: Rework core to use workqueue API

Remove the homegrown sensor delayed work API in favor of using the
system-wide workqueue.  Drivers still have the option of using their own
fiber.

In a second step, drivers can be refactored to start and use their own
workqueue.

Change-Id: I70dea6fc2abcbc9e04ac1ed3c837483a3d3c4424
Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>