Fix default handling in LoadBalancer::getRandomNonLagged()

Fix default handling in LoadBalancer::getRandomNonLagged()

* Previously, setting "max lag" greater than MAX_LAG had no effect.
  Although that only affects really large (11+ seconds) values for
  a production setup, it could be useful for testing.
* Also make it so INF works, which disables auto read-only mode
  altogether. This is useful for testing bugs with REPEATABLE-READ
  and replica DB usage. Setting $wgDBServers two have two entries
  to the same single DB makes LB treat it like a cluster, thus
  DB_SLAVE uses a second connection (and snapshot). Using the INF
  "max lag" value avoids read-only mode in this testing setup.

Change-Id: I4c17fe8f812307e90bcc24820f57c6a1257e9ad0