Brad Fitzpatrick 484b7fc9a3 derp, cmd/derper: add frameWatchConns, framePeerPresent for inter-DERP routing
This lets a trusted DERP client that knows a pre-shared key subscribe
to the connection list. Upon subscribing, they get the current set
of connected public keys, and then all changes over time.

This lets a set of DERP server peers within a region all stay connected to
each other and know which clients are connected to which nodes.

Updates #388

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
2020-06-03 08:03:29 -07:00
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