remove *Ref.copy/append/grow [LUCENE-4675]

These methods are dangerous:

In general if we want a StringBuilder type class, then it should own the array, and it can freely do allocation stuff etc. this is the only way to make it safe.

Otherwise if we want a ByteBuffer type class, then its reference should be immutable (the byte[]/offset/length should be final), and it should not have allocation stuff.

BytesRef is none of these, its like a C pointer. Unfortunately lucene puts these unsafe, dangerous, trappy APIs directly in front of the user.

What happens if i have a bug in my application and it accidentally mucks with the term bytes returned by TermsEnum or the payloads from DocsAndPositionsEnum? Will this get merged into a corrupt index?

I think as a start we should remove these copy/append/grow to minimize this closer to a ref class (e.g. more like java.lang.ref and less like stringbuilder). Nobody needs this stuff on bytesref, they can already operate on the bytes directly.

---
Migrated from [LUCENE-4675](https://issues.apache.org/jira/browse/LUCENE-4675) by Robert Muir (@rmuir)

Start by locating the BytesRef class and its copy, append, and grow methods, then search the repository for their callers. Review the existing tests covering BytesRef and its consumers; done means the unsafe APIs are removed and the relevant test suite still passes.

Written by the indexing model from the issue text.