Server should allow multiple users stored in the same database to authenticate on the same connection

The server currently only allows you to authenticate one user at a time from a given database per connection. Now that we support storing users in one database and granting them access to other databases, here's a reasonable use case:

1. I create users "X" and "Y" in the "users" database
2. I grant "X" access to some databases
3. I grant "Y" access to some other databases
4. I want to authenticate as both "X" and "Y" on the same connection to get access to both sets of databases

This is in contradiction to SERVER-8144 which requests that logging on as "Y" (after "X") will automatically log off "X". But I think the behavior being requested in this JIRA is more logical and useful.

A related change would be that the logout command should have an additional parameter called "username".

This would be slightly backward breaking if a program was counting on the previous behavior, but only in the sense that they would have slightly more privileges than if "X" was automatically logged out.