Improve remote forwarding diagnostics

[barfle]

Summary

• Allow port-forward and SSH tunnels to allocate a separate remote channel for each local client connection.
• Track active clients and first-byte latency per forwarded connection in the live dashboard.
• Add debug request logging around device history API calls to make endpoint issues easier to diagnose.

Test plan

• /tmp/dataplicity-cli-test-venv/bin/python -m pytest