Better support for partial buffer reads/writes in translog infrastructure

Some IO api can return after writing & reading only a part of the requested data. On these rare occasions, we should call the methods again to read/write the rest of the data. This has cause rare translog corruption while writing  huge documents on Windows.

Closes #6441

PS. I'll add java docs to the new Channels util class, but wanted to start the review process.
Ah - I get it :) final to prevent extending, private ctor to replace the abstract. Coming up.