Custom rdds: rdds package restructure

Main change is refactoring the rdds package to have smaller module files, with many classes migrating down into an rdds.fileio subpackage.

Also an update to the scala FixedLengthBinaryRecordReader to fix an issue occurring when a read from a file input stream did not return as many bytes as requested. This was leading to data corruption when reading from s3n. This is the only code change in this PR beyond renaming and moving files.