RUBY-686 issue_authentication, issue_logout and pool cleanup

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