Bridge: make some headers submissions free (#4102)

supersedes https://github.com/paritytech/parity-bridges-common/pull/2873

Draft because of couple of TODOs:
- [x] fix remaining TODOs;
- [x] double check that all changes from
https://github.com/paritytech/parity-bridges-common/pull/2873 are
correctly ported;
- [x] create a separate PR (on top of that one or a follow up?) for
https://github.com/paritytech/polkadot-sdk/tree/sv-try-new-bridge-fees;
- [x] fix compilation issues (haven't checked, but there should be
many).

---------

Co-authored-by: Adrian Catangiu <adrian@parity.io>

bridges/testing/environments/rococo-westend/spawn.sh

@@ -59,12 +59,12 @@ if [[ $init -eq 1 ]]; then
 fi
 
 if [[ $start_relayer -eq 1 ]]; then
-  ${BASH_SOURCE%/*}/start_relayer.sh $rococo_dir $westend_dir relayer_pid
+  ${BASH_SOURCE%/*}/start_relayer.sh $rococo_dir $westend_dir finality_relayer_pid parachains_relayer_pid messages_relayer_pid
 fi
 
 echo $rococo_dir > $TEST_DIR/rococo.env
 echo $westend_dir > $TEST_DIR/westend.env
 echo
 
-wait -n $rococo_pid $westend_pid $relayer_pid
+wait -n $rococo_pid $westend_pid $finality_relayer_pid $parachains_relayer_pid $messages_relayer_pid
 kill -9 -$$