internal/poll: re-use RawSockaddrAny buffers across requests on windows

internal/poll: re-use RawSockaddrAny buffers across requests on windows

We were re-allocating a new RawSockaddrAny on every UDP read/write.
We can re-use them instead.

This reduces the number of allocs for UDP read/write on windows to zero.

Co-authored-by: David Crawshaw <crawshaw@golang.org>
Change-Id: I2f05c974e2e7b4f67937ae4e1c99583e81d140af
Reviewed-on: https://go-review.googlesource.com/c/go/+/361404
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>