Allow maxStalenessSeconds values as small as 40

mongos 3.4 adheres to the original Max Staleness Spec, which said that maxStalenessSeconds must be at least twice heartbeatFrequencyMS. mongos's replica set monitoring period is 30 seconds (right?) that means its smallest maxStalenessSeconds is 60.

Update for a recent change to the Max Staleness Spec: maxStalenessSeconds must be at least heartbeatFrequencyMS + idleWritePeriodMillis, which are 30 and 10 seconds. Therefore the smallest maxStalenessSeconds is 40.