Support for `keep-alive` header

[The-EDev]

While Crow supports Connection: Keep-Alive, it ignores the optional keep-alive header which defines parameters for the persistent connection. The specification says that there are no parameters defined. But MDN mentions 2: timeout and max.

Crow currently uses the default server timeout for keep-alive connections. and outside of debug mode, I don't believe it does any tracking of the number of requests. Therefore a full implementation will require modifying the dumb_timer_queue to support custom times, and adding request and per-connection request tracking.

A partial implementation can have Crow send its own keep-alive header while ignoring the ones potentially sent by the client.