runtime: add concurrent write checks to swissmap

runtime: add concurrent write checks to swissmap

This is the same design as existing maps.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: I5f6ef5fea1e0f0616bcd90eaae7faee4cdac58c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/616460
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>