Remove nbrPatchSets from Change

The nbrPatchSets counter was used to generate the next PatchSetId.
Instead, find the first reference after the currentPatchSetId() that
does not exist by examining the references themselves.

Also in a few locations, the use of currPatchSetId() was corrected
to be currentPatchSetId().

Change-Id: I06bcf52798f1dad1ea602f914784dfcbc7d97086
8 files changed