Support persistent connection, even in single thread mode (i.e concurrency=none).

Warning: as there is no concurrent request handling in single threaded mode,
            it is recommended to either set the keep_alive_timeout to a small value,
            or disable persistent connection by setting max_keep_alive_requests to 0.
Change the default keep_alive_timeout from 15 to 5 seconds.
Accept -1 as value of max_keep_alive_requests to have unlimited number of request in the same persistent connection.

examples/websocket/ws.ini

@@ -2,11 +2,11 @@ verbose=true
 verbose_level=INFORMATION
 port=9090
 max_concurrent_connections=100
-keep_alive_timeout=35
+keep_alive_timeout=2
+max_keep_alive_requests=-1
 max_tcp_clients=100
 socket_timeout=30
 socket_recv_timeout=5
-max_keep_alive_requests=300
 
 is_secure=false
 secure_certificate=ca.crt