Lift DRAM transfer out of websocket

[pfmooney]

During a migration, the contents of guest DRAM must be transferred from source to destination. It is, by orders of magnitude, the largest thing which must be transferred during said process. Today it is transmitted over the same websocket used for other migration data. To avoid incurring the overhead (copying, encoding, etc) for that large data collection, propolis-server should transfer DRAM contents over a dedicated (TLS, for security) socket.