Add backend ID when exposing circuitid with HiddenServiceExportCircuitID

Currently Tor can expose a circuitid to backends with directive HiddenServiceExportCircuitID using the HAProxy proxy protocol, but because there could be multiple Tor instances accessing the same backend (e.g. onionbalance) circuitids could collide.

I propose a config directive HiddenServiceExposeInstanceID, which enables users to specify a tor instance ID which can be placed instead of dead:beef:4dad.

If this seems ok, I’ll whip up a simple patch.