Store current unit stack in thread local storage

[eagletmt]

Since Rails 4.0, Rails itself became thread-safe and it enables
multi-threading by default in production and test (i.e., it doesn't
insert Rack::Lock middleware).

[r7kamura]

Thanks, it's a reasonable solution to use Thread.current.

[r7kamura]

[nitpick & IMHO] We might want to use :"Chanko::Function.units" than :chanko_unit to help developer find its origin, unless there is any special reason to use it.

[eagletmt]

Agreed. Changed it.