chore: Minor Connection.Manager and Consumer cleanup and flake reduction

[msfstef]

• Removes ConnectionBackoff struct, scope is small and it's verbose (@robacourt nits are good we like them)
• Uses the State struct everywhere - since we have it it makes sense to use it
• Remove ShapeCache dependency in Consumer as it already has ShapeStatus available
• Uses Consumer.whereis in the Consumer tests for allowing Mox assertions for consistency
• Fixes the Consumer test flakes by waiting for the set_snapshot_started in the setup before running the tests, as the tests "override" some Mox expectations and allowances set in the setup if the snapshot is not ready before they get defined.

One thing that really worked for me that I hadn't tried before, is to use a tool like stress (e.g. stress -c 10) to max out the resources used by my machine while running tests to simulate a lower resource system that the tests on CI run on. I managed to consistently reproduce the consumer test flakes that way and debug them.

No changeset cause this is all refactoring and test fixing

[icehaunter]

Nice refactorings, love it! Please don't merge this until Garry's #2273 is in because I think there are conflicts that should go in first

[robacourt]

One thing that really worked for me that I hadn't tried before, is to use a tool like stress (e.g. stress -c 10) to max out the resources used by my machine while running tests to simulate a lower resource system that the tests on CI run on. I managed to consistently reproduce the consumer test flakes that way and debug them.

Great tip, thanks! I've added it here: https://electric-sql.slab.com/posts/sporadically-failing-elixir-tests-pfyia5on