Support minimum idle connections for HttpClient

Currently the keep-alive pool within HttpClient is only increased when there is an active request and there are no extras in the pool. When using Vertx HttpClient in a SSL WAN deployment, keeping the TCP and TLS handshake out of the way of a customer request is preferable.

In such the same way that HikariCP (https://github.com/brettwooldridge/HikariCP) supports a minimum number of idle connections. I think ideally this is more of a percentage with a defined minimum and maximum that is dynamic in the middle.

Even if you didn't support this full model in the beginning, just supporting an easy mechanism for someone on the outside to create additional connections (w/out having to do some kind of fake request to a real URL) would allow someone to implement this entirely on their own. This would also allow for situations where you know you are going to start failing over a ton of requests to a remote location, you could start to warm up connections. So that when you do the switch, the first few hundred customers aren't paying all the handshake penalties.