add shuffling support (PR 6 of N)

[bkmartinjr]

Add shuffling per the original CZI API - a random gather of small contiguous regions as a single I/O buffer, followed by a per-I/O batch shuffle.

The primary deviation from the CZI implementation:

• add explicit epoch support, so that the shuffle order by epoch changes
• fix RNG handling so that all worker processes agree on the random shuffling state (ie., ensure same shuffle across all partitions).