Resumable queries with sortkey

Gerrit allows resumable change list queries by passing in a "sortkey"
 associated with a given change, which is now saved and retreived from
 the database when making change list queries. The sortkey allows
 providing the server with the change we are up to regarding a given
 query.
GerritURL is parcelable so it can be stored in intents.
SyncProcessor now preserves the original GerritURL instance that was
 passed in.

Change-Id: I1adc924d7c6bd7be79077c2a423f6185923cfc10