Add an UpdatePeers method to the cache. This allows us to support netmap peer deltas,
by allowing just the peers to be updated in an existing cache. As a safety check, reject
an update if there was no base netmap data to apply a change to.
Then, when processing peer mutations in the backend, capture any changes that should
be applied to the cache and update it, if one is enabled.
Updates #12542
Change-Id: I2f8790a8fdc5e85fce6700ba4821a8cb10dddffa
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>