Base storage plugin framework

The "Easy" framework allows third-parties to add format plugins to Drill with moderate effort. (The process could be easier, but "Easy" makes it as simple as possible given the current structure.)

At present, no such "starter" framework exists for storage plugins. Further, multiple storage plugins have implemented filter push down, seemingly by copying large blocks of code.

This ticket offers a "base" framework for storage plugins and for filter push-downs. The framework builds on the EVF, allowing plugins to also support project push down.

The framework has a "test mule" storage plugin to verify functionality, and was used as the basis of an REST-like plugin.