OAK-1645 introduced support for reads from secondaries under certain
conditions. The current implementation checks the _lastRev on a potentially
cached parent document and reads from a secondary if it has not been
modified in the last 6 hours. This timespan is somewhat arbitrary but
reflects the assumption that the replication lag of a secondary shouldn't
be more than 6 hours.
This logic should be optimized to take the actual replication lag into
account. MongoDB provides information about the replication lag with
the command rs.status().