Reindex all jobs take too long to start

[CGillen]

Descriptive summary

Currently a reindex is starting and it has taken an unacceptably long time to spool up. It claims 5675709.1ms to query the head of Fedora (though it took closer to several days) and has now been running for 21272:51min, or just under 15 days, without finishing partitioning the URIs. Before any jobs can be submitted to SideKiq, this partitioning has to be completed, so no real work has begin and we're 15 days behind content that would need to be reindexed.

We need to find a way to reindex as we find URIs to reindexable assets.

Expected behavior

Reindexing jobs are queued as URIs are found, not after all URIs are found and partitioned.
Alternatively, reindexing is just generally sped up.

Related work

#2760
#2651