DRILL-3177: Part 2, various Mongo plugin enhancements
- Fix issues with initial Mongo 3 update patch
- Update mongo connection management so we don't generate infinite MongoClients.
- Move from static class to management inside Storage Plugin.
- Increase limit on number of connections.
- move mongo cursor initialization to batch reading, so a query won't block indefinitely on this code
- Update mongo driver version to 3.0.2
- Update host access to use all hosts rather than just master
- Remove references to no longer used UnknownHostException
