Add support for persistent outgoing connections

[Marak]

Some services ( like irc or twitter ) require a persistent outgoing connection.

Generally, there should be a lock on this connection if it's going to be created in response to an HTTP request. Additional HTTP requests to the endpoint after connection should result in a streaming write / response.

We can't guarantee the connection will always be maintained, but it should stay alive most of the time and reconnect immediately on disconnects.

[Marak]

Took a first attempt at this, but was not happy with the implementation.

I'm still not 100% certain I want to add this API feature, as it will significantly increase the complexity of the system.

Will keep the issue open for now.