Geo: allow sync retry logic on the secondaries to be disabled

Currently, up to 50% of the Geo backfill scheduling capacity is taken up by scheduling retries of failed items.

In cases where failures are expected, these retries unnecessarily slow down initial backfill. Let's avoid doing any retries until all objects have been tried at least once.

This is particularly an issue on the Geo testbed, but it could help our customers too. I don't think it's necessary for GA though.

/cc @jramsay

Original suggestion: there should be controls to disable the retry logic; we should have a slider for repositories, and another for files (LFS objects+attachments).