Introduce LanguageServerWrapper#getServerCapabilitiesAsync

[angelozerr]

I created the issue because I wanted to do that since long time (because code was copied/pasted from LSP4E and I had created an issue at eclipse/lsp4e#628)

and LSP4E start to fix that eclipse/lsp4e#1098

I create the issue to remember that it should be good to use getServerCapabilitiesAsync