internal/poll: simplify WriteMsg and ReadMsg on Windows

internal/poll: simplify WriteMsg and ReadMsg on Windows

Let newWSAMsg retrieve the socket from the sync pool for unconnected
sockets instead of forcing the caller to do it.

Change-Id: I9b3d30bf3f5be187cbde5735d519b3b14f7b3008
Reviewed-on: https://go-review.googlesource.com/c/go/+/704116
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>