Move "refs/..." constants to a RefNames class

This class lives in gerrit-reviewdb because "refs/changes/" needs to be
referenced from PatchSet in reviewdb/client (and other UI classes).

Rename constants to all use REFS_* instead of being inconsistent.

Change-Id: I76e6ee7e63a4549b51a43698fb50b54a19dabfc6
40 files changed