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/node/eviction.h

@@ -18,7 +18,7 @@ typedef int64_t NodeId;
 struct NodeEvictionCandidate {
     NodeId id;
     std::chrono::seconds m_connected;
-    std::chrono::microseconds m_min_ping_time;
+    NodeClock::duration m_min_ping_time;
     std::chrono::seconds m_last_block_time;
     std::chrono::seconds m_last_tx_time;
     bool fRelevantServices;