internal/poll: don't use stack-allocated WSAMsg parameters
WSAMsg parameters should be passed to Windows as heap pointers instead
of stack pointers. This is because Windows might access the memory
after the syscall returned in case of a non-blocking operation (which
is the common case), and if the WSAMsg is on the stack, the Go
runtime might have moved it around.
Use a sync.Pool to cache WSAMsg structures to avoid a heap allocation
every time a WSAMsg is needed.
Fixes #74933
Cq-Include-Trybots: luci.golang.try:x_net-gotip-windows-amd64
Change-Id: I075e2ceb25cd545224ab3a10d404340faf19fc01
Reviewed-on: https://go-review.googlesource.com/c/go/+/698797 Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>