• Add echo client and server to the distibution.
• Add MD5 client and server to the distibution.
• Fix http_client: check command-line arguments better, prevent crash.
• Fix IETF conn: can_write_ack() should only care about APP PNS.
• Client: delay stream creation until handshake succeds.
• Reset HTTP stream whose write end is closed prematurely.
• Fix tickable(): mirror behavior of tick() wrt buffered packets.
• Log reason why engine is tickable.