wgengine/magicsock: make peerMap also keyed by NodeID

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In prep for incremental netmap update plumbing (#1909), make peerMap
also keyed by NodeID, as all the netmap node mutations passed around
later will be keyed by NodeID.

In the process, also:

* add envknob.InDevMode, as a signal that we can panic more aggressively
  in unexpected cases.
* pull two moderately large blocks of code in Conn.SetNetworkMap out
  into their own methods
* convert a few more sets from maps to set.Set

Updates #1909

Change-Id: I7acdd64452ba58e9d554140ee7a8760f9043f961
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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