Use Control EP for socket commands

[dberliner]

Device will have to manage synchronization to make sure we don't close before data is sent and similar problems. But we should move towards the more standard procedure of using a control endpoint to communicate these short (preferable low latency) commands. Rather than stacking every command into a bulk transfer, the bulk EP will become exclusively for handling write operations between the device and host.

Instead of using variable length payload packets for commands like CONNECT, use fixed size command packets up to 64-bytes long. Dispense with the immediate type and make all bulk transfers payload.