Add reopen method in PerThreadPKLookup

[vsop-479]

Description

Reuse loaded segments' termsEnum and postingsEnum.

[vsop-479]

@jpountz

Please take a look when you get a chance.

[github-actions]

This PR has not had activity in the past 2 weeks, labeling it as stale. If the PR is waiting for review, notify the dev@lucene.apache.org list. Thank you for your contribution!

[vsop-479]

@jpountz
Please take a look when you get a chance.

[vsop-479]

I can't figure out what can we reuse from prior PostingsEnum.

If we want reuse prior PostingsEnum's docBuffer( if it still stay in first block), we should not reset docBufferUpto to BLOCK_SIZE (maybe set to 0?) when prior docStartFP equals current's, which will result in refillDocs.

For the situation of PK, every PK has different PostingsEnum, prior PostingsEnum is mostly useless if reuse means reuse same PostingsEnum loaded docBuffer.

@jpountz Please correct me if I am wrong.

[jpountz]

I wanted to look at your PR locally to understand what it did. It looked good to me but I saw a few simplifications I wanted to make, such as not caching null TermsEnum, since returning null Terms is not expensive on a reader. I felt free to push to your branch, I hope you don't mind. The change looks good to me.

[vsop-479]

This change looks clearer, Thanks @jpountz .

[github-actions]

This PR has not had activity in the past 2 weeks, labeling it as stale. If the PR is waiting for review, notify the dev@lucene.apache.org list. Thank you for your contribution!