Enforce maxStalenessSeconds >= 90

Changes to max staleness spec:

The $readPreference option "maxStalenessSeconds" is no longer allowed to be double, it's once again only int32 or int64. (This change can be put off, need not block 3.4.)

The minimum is now 90 seconds. (3.4 blocker.)

See the "mongos tests" section here:

https://github.com/mongodb/specifications/blob/master/source/max-staleness/max-staleness-tests.rst#mongos

cc schwerin misha.tyulenev