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
@s1monw Added a test in https://github.com/elasticsearch/elasticsearch/commit/0c8f5ac129e5eb164a725b297937f58a9ae9a273