The current version only support SSE implementation and the
multi-threaded scenario has not been tested yet. And the other
implementations in the dcurl_entry are removed in the developing
period. The dcurl_entry will be more readable and easy to add
new implmentation in the future.

Testing steps:
* make
* ./build/test-dcurl

A more general way to iterate the registered PoW implementations

The use of the semaphore can avoid unnecssary spinlock, which results in
about 5% performance loss.

Since the dcurl would be compiled for gpu or non-gpu, multi-pow test
case for gpu specifically is unnecessary.