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.
That can be quite a bit of diff. A handy utils you can suggest to do the proper thing? (up to 20 bytes dump the array,  o.w. just the mismatches / lengths etc.)