Make host memory allocations coherent by default.

Associated change is to optimize event recording so it uses
agent-scope releaes (since it was only using system-scope release
to support non-coherent host mem).

Flags and environment variables exist to obtain previous behavior
if desired.  Options are documented in new performance guide.

README.md

@@ -32,6 +32,7 @@ HIP releases are typically of two types. The tag naming convention is different
 - [HIP Runtime API (Doxygen)](http://rocm-developer-tools.github.io/HIP)
 - [HIP Porting Guide](docs/markdown/hip_porting_guide.md)
 - [HIP Porting Driver Guide](docs/markdown/hip_porting_driver_api.md)
+- [HIP Programming Guide](docs/markdown/hip_programming_guide.md)
 - [HIP Profiling ](docs/markdown/hip_profiling.md)
 - [HIP Debugging](docs/markdown/hip_debugging.md)
 - [HIP Terminology](docs/markdown/hip_terms.md) (including Rosetta Stone of GPU computing terms across CUDA/HIP/HC/AMP/OpenL)