[DB2] Finalize MVP and simplify DRDA protocol classes

[aguibert]

Now that the DB2 module has a good amount of functionality and is getting close to a minimum viable product, I need to go over the codebase and resolve all TODO comments

Also, the code in the io.vertx.db2client.impl.drda package has been intentionally left in a messy state. The goal here was to keep leftover commented out code from the initial Apache Derby port in place so that we could easily reference the "good code" when things go wrong. Now that the codebase is more stable and has a good amount of test coverage, we can finally go through and refactor the DRDA codebase and use more modern APIs and constructs.