Bug 58926: Keep track of known commits in rt server

Tell rt clients to restart if using old version.

Note that this only discards new/old commits depending on whether the server
knowns them, not depending on the commit's timestamp.

Change-Id: I8ac54c3479bd73a246948b3b19095701e5c33c39
Merge "Bug 58926: Keep track of known commits in rt server"