Get rid of extra indirection on tablet metadata

The original design for tablet metadata used an extra indirection of "superblock" which pointed to two "master blocks" which alternated. This was under the assumption that the block manager would support re-writing a block and re-using the same block ID. That got thrown out for other reasons, and now the indirection ends up being somewhat superfluous.

Let's simplify the tablet metadata and just put the superblock directly in a "pb-container" file.