Create chef_config module

Extracts previously private functionality from chef_s3_ops
Harmonized return types for S3 op functions

Now both checking and deleting return tuples with lists of checksums
for all of the 'ok', 'missing', 'timeout', and 'error' states.
Tests for type-safe config retrieval
Address more review comments
More refactoring: chef_parallel

Extract timeout-aware parallelization code and make it fully generalizable for use elsewhere in Erchef.

Re-do chef_s3_ops to account for it.