[hotfix] send healtchecks in sequence

context:

• still getting a lot of false negative healthchecks, still unsure why

changes:

• let's try sending the healtchecks out in sequence instead of all in one parallel burst to see if that helps!

side-effects:

• sending them in sequence lets us write a more deterministic integration test (hopefully fixing a chronically flaky one!)
• reverts commit e957bdc8: it was confusing to register the callback for a healtcheck before sending it, and it didn't fix any problems, so let's revert the commit to eliminate unnecessary cognitive load! :)