Phase 1: Stop compiling "Responding to BEGIN cells" in --disable-module-relay
We want to stop compiling the code that makes tor respond to BEGIN cells, when the relay module is disabled.
(We already disable ORPort, so the functionality is not available.)
At each stage, we should work to minimize layer-violations: there should generally not be calls from src/core/ into relay-specific code, and we should plan to refactor as needed to minimize them. We can reduce layer violations in parallel with the above.