Support persistent connections (aka HTTP/1.1 Keep-Alive)

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Refs #5

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Small progress update: I've started working on this a while back and actually have a working prototype. The implementation is actually quite simple: Once a complete response is received, put the now idle connection into a list of idle connections. If a new outgoing request is to be sent, check if one idle connection to this destination already exists. After a very short grace period (defaults to a millisecond or so, possibly configurable), close the connection if no new outgoing request is sent.

This way, consumers of this library do have to take care of this feature. Also, by using a very short grace period, the loop does not keep running noticeably longer than without this feature. This doesn't matter for long running scripts, but it's important for short running scripts to not appear to be "blocked".

The implementation depends on proper message boundary detection, as such it is currently blocked by #136.

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I have to close this one as per #152 now that future development will focus on https://github.com/reactphp/http instead.

I've migrated this ticket to reactphp/http#39.