Allow more than one connection in WiFiSPIServer

[JiriBilek]

The WiFiSPIServer class allows only one incoming connection, although ESP8266 has no such limitation (in fact, there is obviously an limitation but is higher than one) . The limit is forced by the _sock array that serves as binding information between master and slave.
Discussed in #13

[JiriBilek]

Strange, many internet sources say the ESP8266 can make 4 (or 5) connections at a time. I have just tested 32 concurrent TCPIP connections coming from 3 distinct IP addresses (not having more computers here).

[m0rt3nlund]

I have read 8 maximum concurrent, but also that this is configurable :-) Mvh Morten Lund +47 98203842

…

On 12 Jan 2019, at 11:18, Jiri Bilek ***@***.***> wrote: Strange, many internet sources say the ESP8266 can make 4 (or 5) connections at a time. I have just tested 32 concurrent TCPIP connections coming from 3 distinct IP addresses (not having more computers here). — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub, or mute the thread.

[JiriBilek]

I did more tests - 16 concurrent clients running fine.
https://gist.github.com/JiriBilek/7bdde1863a3340ad58cbebbcc080583e