ipn/ipnlocal: fix cache update for peers deleted by netmap deltas (#20851)

After a netmap delta is applied, we scan the mutations for affected peers and
update the cache (if enabled) for those peers. For removals in particular, we
were relying on the node backend to resolve node IDs (provided by the delta
mutation) to stable IDs.

Prior to 65fd320a this happened to work because the node backend would hold on
to all the peers mentioned by the previous full netmap, even after applying
deltas. But that was essentially accidental, and once we fixed it not to do
that, these lookups no longer worked. We need the stable ID, since that is how
the cache is keyed, and now that they're no longer pinned, we were not properly
evicting removed peers from the cache.

To fix this, capture removed peer stable IDs while applying mutations to the
node backend, instead of trying to look them up afterward.

Updates #20796

Change-Id: I14ded78eaf9657645f0869a52460fd3cd86edba6
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>