LUCENE-6006, here's another pre-cursor for LUCENE-6005
... because I think it's important to nail down Lucene's low-schema
(FieldType/FieldInfos) semantics before adding a high-schema.
IndexableFieldType.indexed() is redundant with
IndexableFieldType.indexOptions() != null, so we should remove it,
codecs shouldn't have to write/read it, high-schema should not configure it, etc.
Similarly, the FieldInfo.indexed bit is redundant, so I removed it, but I
left the sugar API (FieldInfo.isIndexed) and implement it as just
checking IndexOptions != null.