Implement better equals() on PostingsFormats where possible [LUCENE-4093]

[asfimport]

As of #5162, we use HashMap to deduplicate per-field configurations, but all PostingsFormats still implement Object.hashCode/equals.

This is fine, but it means more files etc could be created than are really necessary.

---

Migrated from LUCENE-4093 by Robert Muir (@rmuir)