SI-6773 Makes the change in SI-6150 binary compatible with 2.10

DO NOT MERGE INTO 2.11

The back ported fix of SI-6150 1f0e4880ad7ad816fd82c04f6814c5b165f86981
broke binary compatibility. This commit fixes that by removing the
IndexedSeqFactory class and copy/pasting its one method around to
everything that extended it.

DO NOT MERGE INTO 2.11

review @retronym