add ActiveLeavesUpdate, remove StartWork, StopWork (#1458)

* add ActiveLeavesUpdate, remove StartWork, StopWork

* replace StartWork, StopWork in subsystem crate tests

* mechanically update OverseerSignal in other modules

* convert overseer to take advantage of new multi-hash update abilities

Note: this does not yet convert the tests; some of the tests now freeze:

test tests::overseer_start_stop_works ... test tests::overseer_start_stop_works has been running for over 60 seconds
test tests::overseer_finalize_works ... test tests::overseer_finalize_works has been running for over 60 seconds

* fix broken overseer tests

* manually impl PartialEq for ActiveLeavesUpdate, rm trait Equivalent

This cleans up the code a bit and makes it easier in the future to
do the right thing when comparing ALUs.

* use target in all network bridge logging

* reduce spamming of  and