Allow enabling NODELAY sock option

[BenjaminCubbage]

It would be nice to enable the TCP_NODELAY socket option via the Paho client. This is especially useful for systems where a client is running on the same machine as the broker, as the Nagle algorithm can increase latency by an order of magnitude.

[icraggs]

In the version just released, 1.3.16, this is now enabled by default, and can be turned off with a CMake option PAHO_NO_TCP_NODELAY. I expect to make this an API option in 1.4.

[fpagliughi]

Where the option is available, don't forget to enable TCP_NODELAY on your broker, too! (Like I keep forgetting to do when I switch machines).

For mosquitto, in the configuration:

set_tcp_nodelay true