websocket_api message logging can span multiple lines

[theoreticalbts]

@Gandalf-the-Grey suggests that log lines like:

526454ms th_a       websocket_api.cpp:88          on_message           ] message: {"jsonrpc": "2.0",  
                                         "id": 1  
                                         "method": "get_dynamic_global_properties",  
                                         "params": [],}

should be avoided. The problem is that wdump() outputs the raw string received from the user.