Reuse WebClient for pooling, pipe-lining, multiplexing

Make use of HTTP/1.x pooling and keep alive, HTTP/1.1 pipe-lining, and HTTP/2 multiplexing as supported by WebClient based on HttpClient: https://vertx.io/docs/vertx-core/java/#_http1_x_pooling_and_keep_alive

For a high number of tenants (for example GBV) this will drastically reduce the number of HTTP connections and will reduce the latency resulting from establishing new connections.