Support multiple language servers per document

[sntgluca]

trafficstars

Description

Dear @krassowski , after the improvements achieved with jedi-language-server for autocompletion (#272), I am looking forward to configuring it to work with the code diagnostics features.

Unfortunately the latter doesn't work out of the box and I can't find any useful debugging/troubleshooting message to share.

Would it be possible to get some help to understand what code diagnostics expects and how to include its configuration? Many thanks

Reproduce

1. Install jedi-language-server as described in https://github.com/pappasam/jedi-language-server
2. Create a minimal server setting (See #272 for example)
3. Make a PEP8 mistake in the code

Expected behavior

Expecting seeing the Diagnostic Panel getting populated. Instead nothing happens.

Thanks!