Benchmarks: Move classes into sub-packages to reflect namespace.

Refactoring benchmark.* package organization to better reflect the
various API operations that can be performed on benchmarks.

This commit moves the 'start', 'status', and 'abort' actions into their
own sub-packages, renames a few methods accordingly, and updates some
variable names to reflect the change.

Commit also standardizes all package naming to 'benchmark' from 'bench'.