Revert "concurrency in horizon reading for huge gains, woo"

That was not actually working right, concurrently yielding
to Horizon::Indexer has unexpected results, we had race conditions
and duplication of indexing.

Perf gains unclear, they might have bene artifacts of the
thing totally misbehaving. And actual slowness problem
from Horizon just sometimes hanging too; not worth
anymore time on this, we're gonna have to live with it.

This reverts commit 784686d07ff4c910143e9e6c8eb1365c6b650b03.

Conflicts:
	lib/traject/horizon_reader.rb