ipn/ipnlocal/netmapcache: invalidate digest caches when removing objects (#20839)

Previously, a delta update that drops a peer would not invalidate the digest
cache for that peer after deleting its cache entry. If a subsequent (later)
delta re-adds that peer with the same content, the digest cache would prevent
us from updating the persistent entry.  Add a test to exercise this, and fix
the bug.

Updates #20795

Change-Id: I4a9ef03e8787a330d6395629251ee61ca2221fdd
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>