src: simplify v8 thread pool implementation

This commit drops the semaphore in exchange for a second condition
variable and makes the task ring an array member instead of allocating
it on the heap.  That in turn makes size calculations a little easier
because of the array's fixed size.

PR-URL: https://github.com/node-forward/node/pull/34


Reviewed-By: Fedor Indutny <fedor@indutny.com>