Drop per-target delivery tracking

[snarfed]

...ie Object.undelivered, sent, failed, etc. It's useful, but puts us over the 1MB datastore entity limit and breaks delivery for accounts with lots of followers, and it also generates a lot of datastore write load.

Same with Object.feed, I guess?

[snarfed]

Sigh. This got rid of a lot of our contention rollbacks (deployed at 12-14 ~10:00p PT, ie 3/4 of the way to the right in the top two graphs here):

...but it barely had any effect on our write load:

...and it didn't really even affect our contention failures:

https://console.cloud.google.com/errors/detail/CKbL5KSX98uZHw;time=P1D;locations=global?inv=1&invt=AbkNGg&project=bridgy-federated

https://console.cloud.google.com/errors/detail/CJm_4sDv9O-iKg;time=P1D;locations=global?inv=1&invt=AbkNGg&project=bridgy-federated

I may revert this. 🤷

[snarfed]

Leaving it for now.