refactor: encapsulate flags access for dirty and fresh checks

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

Co-Authored-By: l0rinc <pap.lorinc@gmail.com>

src/test/fuzz/coinscache_sim.cpp

@@ -175,7 +175,7 @@ public:
     bool BatchWrite(CCoinsMap& data, const uint256&, bool erase) final
     {
         for (auto it = data.begin(); it != data.end(); it = erase ? data.erase(it) : std::next(it)) {
-            if (it->second.flags & CCoinsCacheEntry::DIRTY) {
+            if (it->second.IsDirty()) {
                 if (it->second.coin.IsSpent() && (it->first.n % 5) != 4) {
                     m_data.erase(it->first);
                 } else if (erase) {