Pack multiple handshake messages into a single record

[bifurcation]

Right now, mint sends an individual record for every handshake message, which incurs at least 17 bytes of overhead per handshake message (16-byte tag + content type). With this PR, when not in datagram mode, mint will aggregate multiple handshake messages into a single record. There is no change to the behavior in datagram mode.

This PR also adds a new log type pipe, which allows you to log data being sent between client and server.