Added C++11/14 compatible std::shared_mutex (#1095)

Port is based on GCC 7.4 std::shared_mutex implementation

Relates-To: OAM-756
Signed-off-by: Andrey Kashcheev <[email protected]>

Author: andrey-kashcheev

olp-cpp-sdk-core/CMakeLists.txt

@@ -111,6 +111,7 @@ set(OLP_SDK_PORTING_HEADERS
     ./include/olp/core/porting/export.h
     ./include/olp/core/porting/make_unique.h
     ./include/olp/core/porting/platform.h
+    ./include/olp/core/porting/shared_mutex.h
     ./include/olp/core/porting/try_emplace.h
     ./include/olp/core/porting/warning_disable.h
 )