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>
