os.linux: Add support for thread priorities in the KernelStateProvider

os.linux: Add support for thread priorities in the KernelStateProvider

This change tracks a threads priority in the KernelStateProvider. The
KernelThreadInformationProvider provides a new access method returning a
threads priority at a given timestamp.

Change-Id: I4911a75151f45b995ea2000d485d309e4b57313e
Signed-off-by: Christian Mansky <christian.mansky@gmx.at>
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/42152
Reviewed-by: Hudson CI