data-race: `filterRRSlice` function mutates original which it got from the shared cache

[DmitriyMV]

So the chain goes like this:

• In cache.Cache.getIgnoreTTL goroutine gets a pointer to a shared object which contains previous response.
• In cache.Cache.ServeDNS goroutine calls a toMsg method on item to transform shared object into unique response.
• And then cache.filterRRSlice mutates shared object without synchronization.

My best guess is that:

		r.Header().Ttl = ttl
		if dup {
			rs[j] = dns.Copy(r)
		} else {
			rs[j] = r
		}

should be

		if dup {
			copied := dns.Copy(r)
			copied.Header().Ttl = ttl
			rs[j] = copied
		} else {
			r.Header().Ttl = ttl
			rs[j] = r
		}