Support multiple language servers for single file/workspace

[yyoncho]

LSP mode must support multiple LSP servers for single file/workspace similar to what the other implementations(Che/VSCode) already do.

See eclipse-che/che#4609

[TysonAndre]

Possible duplicate of #105

[yyoncho]

@TysonAndre no, #105 is to support different servers for the same mode per workspace. This bug is to support multiple simultaneous servers running in the same file and providing data for the file simultaneously, e.g. running HTML and JS LSP servers in one file.

[yyoncho]

@vibhavp can you share your thoughts? If there are no volunteers to implement this feature I will work on it, but this is not going to happen anytime soon. Since this would require some bit restructuring on lsp-mode side I want to discuss it upfront. I think that it would be good to address some of the lsp-mode flaws related to process management.

[yyoncho]

@vibhavp @MaskRay
I am investigating whether sesman could be a good fit for our usecase which is used by cider for a similar use case. I believe that it could improve the user experience related to visibility what are the currently running servers(check sesman-browser), it will cleanup a lot of code related to session management which complexity is going to increase. Also, it will make the writing of extension against LSP mode much easier compared to current ad-hoc session management with lsp--workspaces hash table.

(cc sesman owner @vspinu)