net: store best block tip time inside PeerManager

And implement 'ApproximateBestBlockDepth()' to estimate
the distance, in blocks, between the best-known block
and the network chain tip. Utilizing the best-block time
and the chainparams blocks spacing to approximate it.

src/net_processing.h

@@ -92,8 +92,8 @@ public:
     /** Send ping message to all peers */
     virtual void SendPings() = 0;
 
-    /** Set the best height */
-    virtual void SetBestHeight(int height) = 0;
+    /** Set the height of the best block and its time (seconds since epoch). */
+    virtual void SetBestBlock(int height, std::chrono::seconds time) = 0;
 
     /* Public for unit testing. */
     virtual void UnitTestMisbehaving(NodeId peer_id, int howmuch) = 0;