refactor: Use NodeClock::duration for m_last_ping_time/m_min_ping_time/m_ping_wait

This refactor does not change any behavior and is needed for a future
commit, to avoid having to add duration casts.

It also improves the docs to better document that this is not a time
point, but a duration.

Also, it uses decltype to explain where the _::max() is coming from.

src/net_processing.h

@@ -52,7 +52,7 @@ static const unsigned int MAX_HEADERS_RESULTS = 2000;
 struct CNodeStateStats {
     int nSyncHeight = -1;
     int nCommonHeight = -1;
-    std::chrono::microseconds m_ping_wait;
+    NodeClock::duration m_ping_wait;
     std::vector<int> vHeightInFlight;
     bool m_relay_txs;
     int m_inv_to_send = 0;