redo security (aka. access control)

Break up monolithic SecurityPlugin API into three parts.

1. Authentication.
2. Authorization.
3. Access Control decision.  Left to ChannelProviders

src/remoteClient/clientContextImpl.cpp

@@ -4164,9 +4164,6 @@ private:
 
         m_channelSearchManager.reset(new ChannelSearchManager(thisPointer));
 
-        // preinitialize security plugins
-        SecurityPluginRegistry::instance();
-
         // TODO put memory barrier here... (if not already called within a lock?)
 
         // setup UDP transport
@@ -4448,11 +4445,6 @@ private:
         }
     }
 
-    const securityPlugins_t& getSecurityPlugins() OVERRIDE FINAL
-    {
-        return SecurityPluginRegistry::instance().getClientSecurityPlugins();
-    }
-
     /**
      * Get channel search manager.
      * @return channel search manager.