Implemented thread service support.

Added a wsrep::thread_service interface to allow application to
inject instrumented thread, mutex and condition variable implementation
for provider.

The interface is defined in include/wsrep/thread_service.hpp.
Sample implementation is provided in dbsim/db_threads.[h|c]pp.

This patch will also clean up some remaining dependencies to
wsrep-API compilation units so that the dependency to wsrep-API
is header only. This will extending the provider support to
later wsrep-API versions.

dbsim/CMakeLists.txt

@@ -12,6 +12,7 @@ add_executable(dbsim
   db_server_state.cpp
   db_simulator.cpp
   db_storage_engine.cpp
+  db_threads.cpp
   dbsim.cpp
 )