Server is shutdown if no file is opened but project is still opened

[ericdallo]

@angelozerr I noticed that if you close any open clojure files, after some seconds it's sent a shutdown request to server even the clojure project is still opened.

I think for clojure-lsp this is bad as clojure-lsp startup usually take 10-30s, depending on the project size, and we don't need to close the connection just because user closed a recent opened file and is navigating the file tree searching for other file.

[angelozerr]

Override LSPClientFeatures#keepServerAlive()

[ericdallo]

That did the trick, thank you!