Remove Fields from TermVector reader related usage

This is a spin-off issue from https://github.com/apache/lucene/pull/180 for Fields class deprecation / removal in TermVector reader usage. As Fields class is generally meant as internal class reserved for posting index, we would like to have some dedicated TermVector abstractions and APIs instead. The relevant discussions are available here:

• https://github.com/apache/lucene/pull/180#pullrequestreview-686320076
• https://github.com/apache/lucene/pull/180#issuecomment-863254651
• https://github.com/apache/lucene/pull/180#issuecomment-863262562
• https://github.com/apache/lucene/pull/180#issuecomment-863775298
• https://github.com/apache/lucene/pull/180#issuecomment-864720190
• https://github.com/apache/lucene/pull/180#pullrequestreview-688023901
• https://github.com/apache/lucene/pull/180#issuecomment-871155896
• https://github.com/apache/lucene/pull/180#issuecomment-871922823

One potential API design for this can be found here https://github.com/apache/lucene/pull/180#issuecomment-871155896