internal/runtime/maps: cleanup seed usage

internal/runtime/maps: cleanup seed usage

Keep only a single seed; initialize it; and reset it when the map is
empty.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: Icc231f70957337a2d0dcd9c7daf9bd3cb4354d71
Reviewed-on: https://go-review.googlesource.com/c/go/+/616466
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>