Add VXI-11 server and improve robustness of socket server

[hb020]

With a VXI-11 server, the device will become much more compatible with various control mechanisms. PyVISA for example.

This PR includes:

• a VXI server (see Interfacing with Python (Pyvisa) requires some work #49)
  • with UDP and TCP port mappers/RPC bind servers
  • and mDNS and lxi publication (both now publish all supported scpi methods)
  • rotating or fixed port VXI-11 TCP server (configured as fixed now to enable mDNS)
  • integration in the web page (connection string, clients, disconnect function)
  • cohabitation with raw socket server (will disable raw socket server when VXI-11 has clients)
• improved robustness of the raw socket server: it locked up when a lot of commands were sent. (see Sending many commands over the raw socket SCPI connection locks up the socket server. #51)
• a pyVISA test script
• additions/corrections in readme (also small doc issue #50)

How it works:

• The vxi servers and rpc bind servers run as independent services (with begin() and loop())
• The vxi server calls upon the SCPI Parser from riden_scpi via an interface class.
• The SCPI parser is kept in riden_scpi to lower the amount of code changes. It has a lockout mechanism: whenever the vxi server serves a client, the raw socket server is disabled. This prevents strange surprises.
• I have not investigated locking with the modbus tcp server, as I do not have the tool set for testing that.

Credits:

The VXI server is heavily inspired by https://github.com/awakephd/espBode, in which I also collaborated.
I made the server "live" in a single directory, with an interface class to the SCPI parser.

Final words

Of course happy to discuss improvements or modifications.

[hb020]

Can't put this PR in draft mode, so doing it via a comment: Hold on, I'll add a small change in lxi discovery in the coming hours.

Done. Ready for merge.