Use relative time for Whisper TTL

[mandrigin]

If a device has skewed clock settings, then the whisper messages it sends are discarded by the TTL check.

From the upstream issue:

Instead of keeping the absolute timestamp of when the envelope was created, we can make TTL counter relative to now() of the current node.

To do that, we create an envelope with a creation time and a TTL (e.g. 50 seconds), we enqueue it.
Just before sending it to another peer via a socket, we subtract now() - creation_time from TTL, and write this value there.
On the receiving node, we update creation_time to now(). Again, just before sending, we subtract now() - creation_time from TTL.

That makes TTL to be smaller and smaller with each hop.

We can keep the logic of discarding messages as it is now.

Upstream issue: ethereum#16134
Status-go issue: status-im/status-go#687