polkadot: disable block authoring backoff on production networks (#2510)

Currently the polkadot node will backoff from block authoring if
finality starts lagging. This PR disables this mechanism on production
networks (polkadot and kusama) and adds a flags to optionally force
enabling it.

polkadot/node/test/service/src/lib.rs

@@ -82,6 +82,7 @@ pub fn new_full(
 			is_parachain_node,
 			grandpa_pause: None,
 			enable_beefy: true,
+			force_authoring_backoff: false,
 			jaeger_agent: None,
 			telemetry_worker_handle: None,
 			node_version: None,