replace stringstream zmq parsing with string construction so we don't rely on null terminated messages

[marc1uk]

std::stringstream expects its input to be null terminated.
Unless we guarantee that all zmq messages received are null terminated, this could lead to the stringstream reading past the end of the message and into arbitrary data, risking segfaulting.
This replaces all instances i could find of std::stringstream for zmq message parsing with a std::string construction that takes the message size. A stringstream is subsequently constructed from this string in the couple of instances where the streamer extraction operator is used.

[marc1uk]

Just commenting here so as not to forget: this breaks RemoteControl (and potentially other things).
The issue is constructing strings with a length means they may include null characters as part of their data. Subsequently string comparisons if(mystring=="All") fail, because "All\0" != "All".
I think likely what we want is something that constructs a string from either the number of bytes specified by the zmq message, or the number of bytes until there's a null, whichever is less.
This is easily done with two steps:

std::string val(static_cast<char*>(msg.data()),msg.size());
val = val.c_str();  // or val = val.substr(0,val.find('\0'));

but it would be good to do this in one step if possible.