Introduced WSREP_ASSERT() for public headers.

Public headers may be compiled with different preprocessor
options than compilation units or private headers within a
project. Introduced a macro WSREP_ASSERT() to be mainly used in
public headers. The macro can be enabled by defining
WSREP_LIB_HAVE_DEBUG and is enabled by default for Debug type builds.

dbsim/db_client_service.cpp

@@ -21,6 +21,8 @@
 #include "db_high_priority_service.hpp"
 #include "db_client.hpp"
 
+#include <cassert>
+
 db::client_service::client_service(db::client& client)
     : wsrep::client_service()
     , client_(client)