tmf: Make Pair utility class non-null and immutable

tmf: Make Pair utility class non-null and immutable

Nullable generic types summon all sorts of hells with Java 8
type annotations. Making this class immutable and non-null
makes it much easier to handle.

Change-Id: I58007eaa488db337d0f34698e3144f232ee3c784
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/37517
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>