This simplifies some netmon code in prep for other changes.
It breaks up Monitor.debounce into a helper method so locking is
easier to read and things unindent, and then it simplifies the polling
netmon implementation to remove the redundant stuff that the caller
(the Monitor.debounce loop) was already basically doing.
Updates #9040
Change-Id: Idcfb45201d00ae64017042a7bdee6ef86ad37a9f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
e881c1caec