refactor: encapsulate flags get access for all other checks

No behavior change. This prepares moving the cache entry
flags field to private access.

src/test/coins_tests.cpp

@@ -603,7 +603,7 @@ void GetCoinsMapEntry(const CCoinsMap& map, CAmount& value, char& flags, const C
         } else {
             value = it->second.coin.out.nValue;
         }
-        flags = it->second.flags;
+        flags = it->second.GetFlags();
         assert(flags != NO_ENTRY);
     }
 }