strip digitizer stores tofBin, hitAssociator uses CFpos

With this PR we fully restore the hit association functionality to what it was before CMSSW_6_2_0_pre8.  That is, for the strip tracker we follow the digiSimLink directly back to the simHit.  This is in contrast to the previous fix (PRs 2279, 2285, 3021, 3637), where the match was indirect through the simTrack, and is sometimes ambiguous.
Here we add a bit to the stripDigiSimLink to specify the Low- vs High-Tof collection, which in combination with the detID.subDet allows the collection-relative pointers to be used.  The digitizer supplies this information on creation of the digiSimLink.  The hitAssociator now uses this information.  I hope I've implemented this in a way that preserves the thread-safe behavior of the hitAssociator.