avoid performing garbage collect on each topology update

[murali-reddy]

Fixes: #115

[bboreham]

I think I would be happier if we did a garabgeCollect somewhere.
Maybe do it on the gossipTimer in localPeer ?

[murali-reddy]

Updated PR to perform garbage collect corresponding to topology updates. Also timer is used trigger garbage collect at scheduled interval which will help coalesce topology updates.

[murali-reddy]

addressed review comments. PTAL

[bboreham]

I don't like this - sendPendingGossip() is carefully written to wait long enough already.

You could add a new function in peers_test.go like:

func forcePendingGC(routers ...*Router) {
	for _, router := range routers {
		router.Peers.Lock()
		if router.Peers.pendingGC {
			var pending peersPendingNotifications
			router.Peers.garbageCollect(&pending)
			router.Peers.unlockAndNotify(&pending)
		} else {
			router.Peers.Unlock()
		}
	}
}

then call it as necessary?

[bboreham]

I note we can't run go test -race on this package any more, because TestDeferredShortIDReassignment() needs to create 4096 peers, and each peer now creates two goroutines, and the Go / LLVM ThreadSanitizer has a limit of 8128 goroutines.