o.a.l.codecs should be pruned to abstract apis [LUCENE-4363]

This package exploded in size to a huge number of classes, Adrien
cleaned up things in #5407 but I think we have more to do.

I think it should look more like o.a.l.analysis: ideally the minimal
set of abstract apis: which there are too many of :)

Currently there is some concrete stuff in here: e.g. i think we should just make a blocktree/ package for the blocktree stuff.

---
Migrated from [LUCENE-4363](https://issues.apache.org/jira/browse/LUCENE-4363) by Robert Muir (@rmuir)

Start by reviewing the o.a.l.codecs package and comparing its structure with o.a.l.analysis. Identify which classes are concrete implementations rather than minimal abstract APIs, including the blocktree-related code. Done means the package is pruned to the intended abstract API set and blocktree code is separated into its own package.

Written by the indexing model from the issue text.