x/tools/gopls: allow parallel request processing

[muirdm]

Currently all requests (e.g. signatureHelp, hover, complete, documentHighlight, etc) are processed synchronously, meaning they block subsequent requests/notifications. This includes the time they spend parsing and type checking, which can be significant. In particular, this is wasteful when a new didChange event comes in, since that "should" cancel all in-flight requests for that package, but currently it has to wait for them all to finish.

When gopls is ready for the extra complexity, @ianthehat says that these requests can change to do most of their work asynchronously.