Peter M. Groen / TrueMQTT-cpp

This is a breaking API change.

The whole interface of this library now uses "std::string_view"
instead of "std::string" / "std::string &". In result, we can
now promise to only copy data once thoughout the library.

For subscriptions, this is a copy once to read the data from the
socket into a buffer.

For publish, this is a copy once to write the data in a buffer
to send over the socket.

For publish, this doesn't change the memory footprint, as because
"std::string &" was already taking care of this. For subscriptions
however, it reduces the memory usage by a factor of three. And as
result it helps with the throughput.

This means the socket can be blocking, which makes administration
easier. The drawback is that there is now a queue, including
signalling, between the main thread and write thread. This
consumes a bit more CPU; but in return the main thread is never
blocked.