If the node is bootstrapped with auto_boostrap disabled, it will not
wait for schema sync before creating global keyspaces for auth and
tracing. When such schema changes are then reconciled with schema on
other nodes, they may overwrite changes made by the user before the
node was started, because they will have higher timestamp.
To prevent that, let's use minimum timestamp so that default schema
always looses with manual modifications. This is what Cassandra does.
Fixes#2129.
d6425e7646