internal/jsonrpc2_v2: initialize async before binding

The lsprpc package needs to wait on the newly bound connection to
perform some tear-down, so it must be safe to call conn.Wait from Bind.

Achieve this by switching async from an init pattern to a constructor.
This means moving async fields to pointers, but since they weren't safe
to use prior to initialization anyway this feels correct.

Change-Id: I4f93980915e0b568fbf2db3cd7d062adf06e4b99
Reviewed-on: https://go-review.googlesource.com/c/tools/+/330929
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

internal/jsonrpc2/jsonrpc2.go

@@ -64,10 +64,12 @@ type async struct {
 	errBox chan error
 }
 
-func (a *async) init() {
+func newAsync() *async {
+	var a async
 	a.ready = make(chan struct{})
 	a.errBox = make(chan error, 1)
 	a.errBox <- nil
+	return &a
 }
 
 func (a *async) done() {