Use standard C++ types instead of type aliases:

* Remove ripple::RippleMutex and ripple::RippleRecursiveMutex
  and use std::mutex and std::recursive_mutex respectively.
* Use std::lock_guard instead of std::unique_lock when the
  additional features of std::unique_lock are not needed.

src/ripple/protocol/impl/SField.cpp

@@ -21,6 +21,7 @@
 #include <ripple/protocol/SField.h>
 #include <map>
 #include <memory>
+#include <mutex>
 #include <string>
 #include <utility>