PeerByStableID did an O(n peers) scan, and an upcoming change needs
the same StableNodeID-to-NodeID resolution whenever prefs change (to
resolve the selected exit node for the route manager, which keys
peers by NodeID because that is the identity netmap delta mutations
carry). Maintain a nodeByStableID index alongside the existing
nodeByAddr and nodeByKey indexes, updated on full netmaps and on
delta mutations.
Updates #12542
Change-Id: Id1e5105a7470b02312533f0f46b69e6945cd62f0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>