langserver: Use jsonrpc2.AsyncHandler
langserver: Use jsonrpc2.AsyncHandler

We still need to handle concurrent requests better w.r.t. the ordering of
didOpens/etc. But for now we use the generic async handler.

sourcegraph/go-langserver,lambdalab/go-langserver,sourcegraph/go-langserver,lambdalab/go-langserver,sourcegraph/go-langserver,lambdalab/go-langserver