Add test that ES filterreader getCoreCacheKey() behaves correctly.

Closes #8831.
Stats: Add more fine grained memory stats from Lucene segment reader.

This is a start to exposing memory stats improvements from Lucene 5.0.
This adds the following categories of Lucene index pieces to index stats:
* Terms
* Stored fields
* Term Vectors
* Norms
* Doc values
Tests: Additional test for memory stats api.