#7 Persistent connections

[Nickolas Grigoriadis]

Support for:

HTTP/1.0 "Keep-Alive" header values
- Section 19.7.1 of RFC 2068

HTTP/1.1 "Connection" header values
- Chapter 8 of RFC 2616

Default to no persistence if "Keep-Alive" is not set or HTTP >= 1.1 was not used.

Provide a method of cannibalizing a waiting connection if using a thread-pool approach.