This is a follow-up to #19117, adding a debug CLI command allowing the operator
to explicitly discard cached netmap data, as a safety and recovery measure.
Updates #12639
Change-Id: I5c3c47c0204754b9c8e526a4ff8f69d6974db6d0
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>