Add aliasing to ConnectionManager.

This will be useful in the testsuite for converting production
connections into the testing ones. This provides a useful feature of the
testsuite in 2.x.
Add ability to get connections without aliasing.

This will be used in the testsuite when dealing with import fixtures.
Don't use aliased connections when working with fixtures.

Using aliased connections would break schema reflection as test ===
default.
Fix aliasing in ConnectionManager

You should be able to alias a connection that does not exist to one
that does.
Use Connection aliasing in test suite.

Wire up default connection names in Table classes. By implementing the
defaultConnectionName() method a table can customize the default
connection it will use if one is not provided to the constructor.

Use connection aliasing in the test suite as it provides the same
behavior that ClassRegistry used to in 2.x.