Lift DRAM transfer out of websocket

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.

Start by tracing propolis-server's migration path and the existing websocket transport for guest DRAM and other migration data. The work is done when DRAM contents use a dedicated TLS-secured socket rather than the websocket, with the migration flow still transferring the remaining data correctly; no specific files or tests are named in the issue.