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.

Otherwise, on reconnect a whole bunch of older samples are sent
out.

Also, check if we get a response every ping-request, so we know
the connection with the broker is still good.

Doing it every 100us consumes a lot of CPU, and was not the
intention.

Additionally, increase the read interval to 100ms, as 10ms is using
quite a lot of CPU for not really a good reason other than reacting
quickly on error situations. That really is not the right balance
to strike.

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.

Before this commit, we had one small buffer telling the packet
type and length, and another buffer with the payload. They were
send to the kernel one by one. For small packets, this is a
problem, as NODELAY causes the first buffer to be send on the
IP stack, and the payload after. This increases the IP overhead
for no good reason.

Now instead, already reserve room in the packet to write the
header, and send it as one single unit to the kernel.

send() no longer is blocking, and all sendNNN calls now return
false if the call couldn't be executed.

Additionally, the library now recovers much better from issues,
like unexpected broker disconnects.

This avoids copying the pointers from Client::Impl into Connection,
which is just administrative work. Now we can access Client::Impl,
and have all variables available to us.

Subscriptions are now stored in a tree-like structure, to quickly
find the correct callbacks. This not only reduces the complexity
from O(n) to O(logn), but also doesn't require stuff like regex.

It does however require slightly more memory.

std::move() is doing the right thing either way.

This includes CONNACK and SUBACK.