internal/poll: remove operation fields from FD

internal/poll: remove operation fields from FD

Use a sync.Pool to reuse the overlapped object passed to the different
Windows syscalls instead of keeping two of them in the FD struct.

This reduces the size of the FD struct from 248 to 152 bytes.

While here, pin the overlapped object for the duration of the overlapped
IO operation to comply with the memory safety rules.

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest,gotip-windows-amd64-race
Change-Id: I0161d163f681fe94b822c0c885aaa42c449e5342
Reviewed-on: https://go-review.googlesource.com/c/go/+/704235
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>