Introduce non-locking variant of client_state::cleanup()

The method takes already locked lock object as an argument.
The caller must ensure that the lock object owns the underlying mutex.

Replaced homegrown wsrep::unique_lock with type alias from
std::unique_lock.

src/client_state.cpp

@@ -69,6 +69,11 @@ void wsrep::client_state::close()
 void wsrep::client_state::cleanup()
 {
     wsrep::unique_lock<wsrep::mutex> lock(mutex_);
+    cleanup(lock);
+}
+
+void wsrep::client_state::cleanup(wsrep::unique_lock<wsrep::mutex>& lock)
+{
     debug_log_state("cleanup: enter");
     state(lock, s_none);
     debug_log_state("cleanup: leave");

src/thread_service_v1.cpp

@@ -23,6 +23,7 @@
 #include "wsrep/logger.hpp"
 #include "v26/wsrep_thread_service.h"
 
+#include <cassert>
 #include <dlfcn.h>
 #include <cerrno>