The refState is a set of refs with SHA1s that fingerprints a document
in the ChangeIndex. Before it was represented as a repeated byte[]
field, which callers had to (de)serialize by calling
RefState#{parseStates,toByteArray}.
With this change, the (de)serialization is pushed out to the index
implementations.
Change-Id: Id49ccf3998b27d5c5f40124aaf1e3ad067a59b41