wgengine/magicsock: make peerMap also keyed by NodeID

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>

wgengine/magicsock/debugknobs_stubs.go

@@ -26,3 +26,4 @@ func debugUseDerpRouteEnv() string     { return "" }
 func debugUseDerpRoute() opt.Bool      { return "" }
 func debugRingBufferMaxSizeBytes() int { return 0 }
 func inTest() bool                     { return false }
+func debugPeerMap() bool               { return false }