beast, beast::asio improvements and fixes:

* New maybe_const_t alias for maybe_const
* New asio::enable_wait_for_async for safe cleanup
* New asio::memory_buffer, a managed boost::asio compatible buffer
* shared_handler improvements:
   - Can be 'empty' (no stored handler).
   - Default constructible as 'empty'.
   - Safe evaluation in bool contexts, false==empty
* Fix is_call_possible metafunction:
   - Works on empty argument lists
   - Works with reference types
* Replace SafeBool idiom with C++11 explicit operator bool
* Move IPAddress function definitions to the header
* Move cyclic_iterator to container/
* Remove unused BufferType
* Remove obsolete classes:
   - NamedPipe
   - ReadWriteLock
   - ScopedReadLock
   - ScopedWriteLock
   - LockGuard

src/ripple_data/protocol/SerializedTypes.h

@@ -622,7 +622,9 @@ public:
     {
         return !mIsNative || (mValue <= cMaxNativeN);
     }
-    operator bool () const
+    
+    explicit
+    operator bool () const noexcept
     {
         return !isZero ();
     }