Record reviewer state in notes database

Stop implicitly adding/removing reviewers by adding an arbitrary
PatchSetApprovals with a 0 vote. This is ugly and error-prone,
especially when LabelTypes change.

Instead, use the existing ReviewerState enum, adding a REMOVED type
as a tombstone. When parsing, record these during the walk, and prune
all REMOVED tombstones after the fact.

Change-Id: Iffb0517eb0162eb6cce66bf36d905a6eb60e75da
11 files changed