Improve async copy implementation.

- Add device-side signal waits when transitioning between command classes
(Kernel, H2D copy, D2H copy).
- Support waiting in staged memory copies as well.
- Add several chicken bits to control implementation:
    - HIP_DISABLE_ENQ_BARRIER
    - HIP_DISABLE_BIDIR_MEMCPY
    - HIP_ONESHOT_COPY_DEP
- Refactor signal pool to support efficient deallocation based on
signsequnm.
- Deallocate copy signals on eventSynchronize.
- Improve copy tests, add pingpong.

tests/src/test_common.h

@@ -88,7 +88,7 @@ vectorADD(hipLaunchParm lp,
     size_t stride = hipBlockDim_x * hipGridDim_x ;
 
     for (size_t i=offset; i<N; i+=stride) {
-		C_d[i] = A_d[i] + B_d[i];
+        C_d[i] = A_d[i] + B_d[i];
 	}
 }