Split agd into Go-only and JS-only executables

[JimLarson]

What is the Problem Being Solved?

See #5287.

Description of the Design

From the development plan in #5287

• Add new JSON-RPC connection type to connections.js and have a modified sim-chain speak it by updating “bridge” device callbacks.
• Create new agvm binary and command-line, replacing ag-solo.
  • ag-solo can be a simple script for compatibility
• Create modified “agd start” command to launch agvm and call-to-controller via pipe.
• The naughty intra-process RPC data exchange bits:
  • vm.ReplyToGo(replyPort int, isError int, resp string) int
  • vm.SendToGo(port int, msg string) string
  • daemon.RunWithController(func (needReply bool, str string) (string, error))

Security Considerations

None.

Scaling Considerations

Might increase bridge communication latency and overhead.

Test Plan

Will need to ensure that termination of either side brings down its peer.