remove fieldcache weakmap or at least see what relies on GC for purging today [LUCENE-5177]

If we are registering close listeners why does this need to be weak?

But really i dont care about that, here is what i said to Hoss on the solr mailing list:
> > (In any case: it looks like a WeakHashMap is still used in case the
> > listeners never get called, correct?)
> >
>
> I think it might be the other way around: i think it was weakmap
> before always, the close listeners were then added sometime in 3\.x
> series, so we registered purge events "as an optimization"\.
>
> But one way to look at it is: readers should really get closed, so why
> have the weak map and not just a regular hashmap\.
>
> Even if we want to keep the weak map (seriously i dont care, and i
> dont want to be the guy fielding complaints on this), I'm going to
> open with an issue with a patch that removes it and fails tests in
> `@afterclass` if there is any entries\. This way its totally clear
> if/when/where anything is "relying on GC" today here and we can at
> least look at that\.

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Migrated from [LUCENE-5177](https://issues.apache.org/jira/browse/LUCENE-5177) by Robert Muir (@rmuir), updated Aug 16 2013
Attachments: [LUCENE-5177.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5177/LUCENE-5177.patch)

Start by reviewing the FieldCache implementation and its close-listener handling, then inspect the existing tests and the LUCENE-5177.patch attachment. Determine which behavior currently relies on WeakHashMap garbage collection and whether replacing it with a regular map is safe. Done means the chosen map behavior is covered and tests fail in @afterclass when entries remain.

Written by the indexing model from the issue text.