coins: introduce CCoinsViewCache::ResetGuard

CCoinsViewCache::CreateResetGuard returns a guard that calls
Reset on the cache when the guard goes out of scope.
This RAII pattern ensures the cache is always properly reset
when it leaves current scope.

Co-authored-by: l0rinc <pap.lorinc@gmail.com>
Co-authored-by: sedited <seb.kung@gmail.com>

src/test/fuzz/coinscache_sim.cpp

@@ -401,6 +401,14 @@ FUZZ_TARGET(coinscache_sim)
                 caches.back()->Sync();
             },
 
+            [&]() { // Reset.
+                sim_caches[caches.size()].Wipe();
+                // Apply to real caches.
+                {
+                    const auto reset_guard{caches.back()->CreateResetGuard()};
+                }
+            },
+
             [&]() { // GetCacheSize
                 (void)caches.back()->GetCacheSize();
             },