Vertx 4 Support

Upgrade vertx to 4.0.0. A big shout out to vertx-jooq user doctorpangloss for the
groundwork.
Enhanced PgConverter: previously, PGConverter was only considered when converting from or into a JsonObject. With
the new release you can now convert anything from a io.vertx.sqlclient.Row into your user object. For that reason I've
introduced the new RowConverter. For an example check out the
CommaSeparatedStringIntoListConverter.
Added support of user-types like java.util.List<U> in your POJOs. Checkout the PostgresConfigurationProvider of how to configure it.
Removal of the async driver. There is actually no reason to use this driver over the reactive driver from vertx. It
just adds confusion for initial users and worsens maintainability.
Removal of the CompletableFuture-API. When this project was started, the io.vertx.core.Future was in a bad shape.
Many methods for composition and error handling were missing and made it hard to actually use. In the past couple
of months this has been fixed - making the io.vertx.core.Future-API a first-class choice. In case you really need
the interoperability with CompletionStage/CompletableFuture just call io.vertx.core.Future#toCompletionStage() to
convert it into, or the static method Future.createFromCompletionStage to convert from a CompletionStage.

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