Create propagatable connection, add Transaction accessor

[technical-debt-collector]

Motivation:

Proposed solution to https://github.com/eclipse-vertx/vertx-sql-client/issues/1214 and https://github.com/eclipse-vertx/vertx-sql-client/issues/646

This could probably be improved a great deal, but I've reached a stage with this one where I'd need some more eyes on it. I'm also struggling with running a lot of the tests, so I am somewhat in the dark on whether or not this has broken anything.

Conformance:

You should have signed the Eclipse Contributor Agreement as explained in https://github.com/eclipse/vert.x/blob/master/CONTRIBUTING.md Please also make sure you adhere to the code style guidelines: https://github.com/vert-x3/wiki/wiki/Vert.x-code-style-guidelines