Lower refresh_interval default from 300 to 10 sec

Current default is too high- if a failover occurs, an existing connection won't detect a new PRIMARY for potentially up to 300 seconds.

I propose changing the default to 10 or 5 seconds. See Durran's code here for reference:
https://github.com/mongodb/mongo-ruby-driver/blob/89733bc5cfab5b82228d663c285135f2deea6cd0/lib/mongo/server/monitor.rb#L28-L58