notedb: Rewrite comments to be keyed by RevId

Rather than maintain two separate maps, one for each side, use a
single multimap of SHA-1 (RevId) to PatchLineComment. Callers are
responsible for looking up the RevId they need (e.g. with the
PatchListCache), but in practice they rarely do.

Change-Id: I5597cc6c12a881e7cf7f3c786b9697b4f7b9749f
13 files changed