languageclient: clangd restarts even when all files are from same project

It seems there is some buggy edge case with autostop due to the way manual starting is currently implemented. I have not yet tested this, but I think that this will trigger it:

1. open some C file, clangd starts
2. open some RST file, clangd stops because of autostop feature
3. open some C file, compdb path not changed, server doesn't start

Of course the correct behaviour would be to not stop the server in the first place, so likely the fix is to disable autostop.