New Mojo::UserAgent event "connect"

[elcamlost]

Now it's impossible to distinguish reused connections vs new connections without running MOJO_CLIENT_DEBUG.
But when you write some complex system with Mojo::UserAgent, for example, you have external requests and need to connect to dynamic urls on different servers to serve this requests, it's hard to determine good value for max_connections parameter.

With new event we can compare start event vs connect event and adjust max_connections to our current needs.

I can submit PR if core team find this feature useful