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