[PROVIDED Phase 2] Block Bit Field Allocation of Provided Storage

We wish to design and implement the following related features for provided storage:

1. Dynamic mounting of provided storage within a Namenode (mount, unmount)
2. Mount multiple provided storage systems on a single Namenode.
3. Support updates to the provided storage system without having to regenerate an fsimg.

A mount in the namespace addresses a corresponding set of block data. When unmounted, any block data associated with the mount becomes invalid and (eventually) unaddressable in HDFS. As with erasure-coded blocks, efficient unmounting requires that all blocks with that attribute be identifiable by the block management layer

In this subtask, we focus on changes and conventions to the block management layer. Namespace operations are covered in a separate subtask.