Add batcher cases for NodeEndpoint and NodeDERP that query
NodeStore for current state and send PeerChangedPatch with
both endpoints and DERP region, avoiding full map responses.
Update batcher_test to pass NodeView to change constructors.
Initial work on a nodestore which stores all of the nodes
and their relations in memory with relationship for peers
precalculated.
It is a copy-on-write structure, replacing the "snapshot"
when a change to the structure occurs. It is optimised for reads,
and while batches are not fast, they are grouped together
to do less of the expensive peer calculation if there are many
changes rapidly.
Writes will block until commited, while reads are never
blocked.
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>
