in order for the newenemy test to work, we need to ensure that the
additional reads for namespaces that have been added for namespace
versioning do not re-sync timestamps across cockroach nodes.

by first filling the datastore with a large number of namespaces, we
can pick one that has a range leader on the slow node, so that future writes to the slow node don't involve reads from other nodes

the core of the test is otherwise the same, though extra machinery for
reasoning about range leaders was added