ss: Speed up TreeMapStore#add()

ss: Speed up TreeMapStore#add()

The add() does not need to call contains(), a O(n) operation, to check
if the element was already present. It can simply re-use the return
value of TreeMultimap.put().

Change-Id: Iab1971f6310163099906df0a1702d106e3640316
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/56904
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI