Enable DNS Resolving via TCP in case of Timeout

Hi everyone,

I noticed that Vertx enables Netty DNS Resolving via TCP (see https://github.com/eclipse-vertx/vert.x/blob/4.5.17/src/main/java/io/vertx/core/impl/resolver/DnsResolverProvider.java#L108):
`builder.socketChannelFactory(() -> (SocketChannel) vertx.transport().channelFactory(false).newChannel());`

However, this sets "retryOnTimeout" to "false". It would be nice if Vertx would enable TCP resolving on timeouts - maybe just with some kind of environment variable for toggling, like this:

`builder.socketChannelFactory(() -> (SocketChannel) vertx.transport().channelFactory(false).newChannel(), Boolean.getBoolean("vertx.dns.retryOnTimeout"));`

I'm happy to provide a PR if you approve of this.

Best regards

### Contribution

Me