derp/derpserver: bound dup client sendHistory growth

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When two connections share a node key, they form a dup client set and
noteClientActivity records each sending connection in the set's
sendHistory. It appended on every frame whenever the sender was not the
immediately previous one, and nothing trimmed the slice while both
connections stayed alive. Two connections taking turns sending therefore
grew sendHistory by one *sclient per frame without bound.

Under the default lastWriterIsActive policy nothing ever stops that
growth, so a malicious client (which controls its own node key) or a
buggy one that keeps two connections alive and both sending could leak
server memory, roughly 8 bytes per frame, for the life of the
connection pair.

Record the sender by moving it to the end of sendHistory and dropping any
earlier occurrence, so each connection appears at most once and the slice
stays bounded by the number of connections in the set. This preserves the
existing behavior: the fighting check under disableFighters still runs
before the move and still disables everyone on the first repeat, and
removeClient still promotes the previous speaker from the slice tail.

Fixes tailscale/corp#48884

Change-Id: I06198178e6ab0d7e2f04c1dc4c09eafcb16ace46
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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