Rethink Fields.size() returning -1 [LUCENE-4319]

Fields.size() is allowed to return -1, if size of iterator is unknown. But for all codecs, we know this size and also Fields implements Iterable (see also discussion on LUCENE-4315).

We should think about enfocing it to be defined.

Some things to do:
- Maybe automatically make Fields.size() base class iterate, if not reimplemented (risky!).
- MultiFields.size() must implement it

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Migrated from [LUCENE-4319](https://issues.apache.org/jira/browse/LUCENE-4319) by Uwe Schindler (@uschindler)
Linked issues:
- #5383

Start by reading the Fields and MultiFields APIs, then review the discussion on LUCENE-4315 and the linked issue #5383. Determine whether Fields.size() should be enforced or computed by a base implementation, and ensure MultiFields.size() follows the decided behavior across codecs.

Written by the indexing model from the issue text.