derp: refactor DERP server's peer-gone watch mechanism

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In prep for upcoming flow tracking & mutex contention optimization
changes, this change refactors (subjectively simplifying) how the DERP
Server accounts for which peers have written to which other peers, to
be able to send PeerGoneReasonDisconnected messages to writes to
uncache their DRPO (DERP Return Path Optimization) routes.

Notably, this removes the Server.sentTo field which was guarded by
Server.mu and checked on all packet sends. Instead, the accounting is
moved to each sclient's sendLoop goroutine and now only needs to
acquire Server.mu for newly seen senders, the first time a peer sends
a packet to that sclient.

This change reduces the number of reasons to acquire Server.mu
per-packet from two to one. Removing the last one is the subject of an
upcoming change.

Updates #3560
Updates #150

Change-Id: Id226216d6629d61254b6bfd532887534ac38586c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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