Cache created decorated connection

[MasterOdin]

PR pulls a commit from the 1.x branch, where when generating the decorated connection as part of getDecoratedConnection, since it's likely that each call to the function would always just return the same thing anyway currently, just with the overhead of creating a fresh class each time. This is most beneficial to people using the Query Builder.

As part of this, I've moved getDecoratedConnection to be part of the base PdoAdapter class as it then allows some code sharing, while creating a new getDecoratedConnectionConfig function that gets the individual adapter code.