Remove client-side KeepAlive code

Closing the connection is the server's responsibility. We
also don't need to negotiate KeepAlive, we'll instead just
start sending requests. That makes us faster, and most (if
not all) servers in the wild are HTTP/1.1. If we encounter
a 1.0 server, the sent requests that haven't been served
will simply get ed.
Keep only the previous request for KeepAlive synchronization
Eliminate unused parameters
Rename #post_init to #setup
Refactor Server#setup_connection
Decouple served promise from Request
Reintroduce client request synchronization
Change spec root_response to echo the request URI
Fix parameter smells
Adjust Flay and Flog config
WIP