This adds the `_primary_term` field internally to the mappings. This field is
populated with the current shard's primary term.

It is intended to be used for collision resolution when two document copies have
the same sequence id, therefore, doc_values for the field are stored but the
filed itself is not indexed.

This also fixes the `_seq_no` field so that doc_values are retrievable (they
were previously stored but irretrievable) and changes the `stats` implementation
to more efficiently use the points API to retrieve the min/max instead of
iterating on each doc_value value. Additionally, even though we intend to be
able to search on the field, it was previously not searchable. This commit makes
it searchable.

There is no user-visible `_primary_term` field. Instead, the fields are
updated by calling:

```java
index.parsedDoc().updateSeqID(seqNum, primaryTerm);
```

This includes example methods in `Versions` and `Engine` for retrieving the
sequence id values from the index (see `Engine.getSequenceID`) that are only
used in unit tests. These will be extended/replaced by actual implementations
once we make use of sequence numbers as a conflict resolution measure.

Relates to elastic#10708
Supercedes elastic#21480

P.S. As a side effect of this commit, `SlowCompositeReaderWrapper` cannot be
used for documents that contain `_seq_no` because it is a Point value and SCRW
cannot wrap documents with points, so the tests have been updated to loop
through the `LeafReaderContext`s now instead.