In order to be able to synthesize a new NetMap when a node expires, have
LocalBackend start a timer when receiving a new NetMap that fires
slightly after the next node expires. Additionally, move the logic that
updates expired nodes into LocalBackend so it runs on every netmap
(whether received from controlclient or self-triggered).
Updates #6932
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I833390e16ad188983eac29eb34cc7574f555f2f3
