Disable concurrent Quartz scheduling of indexing  (see #11945)

With Quartz concurrency enabled, the indexer appears to be getting stuck
in a deadlock when one or more thread experiences an OOM error. Since,
currently, only one thread can index at a time anyway this disables
concurrency in Quartz as a workaround.
Merge pull request #2049 from jballanc/disable-quartz-concurrency

Disable concurrent Quartz scheduling of indexer