Avoid returning references that are mutex protected (#297)

* Fix unsafe calls and safeguard WebSocketMessage from being called w/
temporaries

* Use unnamed namespace to express internal linkage

* Avoid returning references that are mutex protected
Motivation for this MR

The antipattern of returning references to mutex protected members was
removed. Since a caller can hold the reference it would make all class
level locking meaningless.

Instead values are returned. The IXWebSocketPerMessageDeflateOptions
class was shrunk by 7 bytes (1 padding + 2*3) after changing the int
members to the used uint8_t; side effects of that were handled.

An inefficient "string -> int" was replaced by standard library. As
seen here http://coliru.stacked-crooked.com/a/46b5990bafb9c626 this
gives an order of magnitude better performance.

ixwebsocket/IXWebSocketPerMessageDeflateOptions.h

@@ -39,8 +39,8 @@ namespace ix
         bool _enabled;
         bool _clientNoContextTakeover;
         bool _serverNoContextTakeover;
-        int _clientMaxWindowBits;
-        int _serverMaxWindowBits;
+        uint8_t _clientMaxWindowBits;
+        uint8_t _serverMaxWindowBits;
 
         void sanitizeClientMaxWindowBits();
     };