Support http connection keep-alive

Supporting HTTP 1.1 Connection: Keep-Alive is essentially a matter of just not closing the connection after sending a response, and ensuring processing resumes when further data arrives over the connection.

With this modification, a 'keepAlive' parameter is added to the Response end function. The caller can choose to keep the connection open by specifying keepAlive = true. Previous behaviour of closing the connection is retained if the parameter is not specified.

Testing performance with small HTTP POST messages sent using a test program (using php curl) to a test server:
* Closing connection each time: 350 requests per second.
* Persistent connection: 3125 requests per second.
Ah, good point! I was/am worried about resource cleanup that is no longer happening [here](https://github.com/reactphp/http/blob/master/src/RequestHeaderParser.php#L31).