internal/jsonrpc2_v2: move the idle timeout handling out of the server

Change-Id: Ia7dedd84171f4fc4818fb00b154c21b04887391f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/305129
Trust: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Ian Cottrell <iancottrell@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/jsonrpc2/jsonrpc2.go

@@ -68,6 +68,15 @@ func (a *async) done() {
 	close(a.ready)
 }
 
+func (a *async) isDone() bool {
+	select {
+	case <-a.ready:
+		return true
+	default:
+		return false
+	}
+}
+
 func (a *async) wait() error {
 	<-a.ready
 	err := <-a.errBox