When receiving disco traffic from a node, mark that node as having been
seen. Should that disco key not be the active use disco key, switch to
that one as being the active key. Additionally, clear states on the
magicsock connection and prepare for sending a new WG handshake whenever
user data is transmitted.
Sets up for:
- Routing TSMP keys directly into magicsock
- Switching the active connection reset mechanism to the optimistic
handshake
- Cleaning up paths into controlClient
Updates #20494
Signed-off-by: Claus Lensbøl <claus@tailscale.com>