RUBY-686 issue_authentication, issue_logout and pool cleanup

**Note:** This depends on #300 so please don't merge until the second pass at reviewing that PR is complete.

This is it! This is the last part of the RUBY-686 m-m-m-monster ticket.

In this pull request:

* Refactor and relocation of issue_authentication logic
* Refactor and reloaction of issue_logout logic
* Wrapper for legacy support of DB#issue_logout and DB#issue_authentication
* Stub for new auth mechanisms
* Addition of check_auths in Mongo::Pool