validation: don't reallocate cache for short-lived CCoinsViewCache

A few temporary `CCoinsViewCache`'s are destructed right after the `Flush()`, therefore it is not necessary to call `ReallocateCache` to recreate them right before they're killed anyway.

* `Flush()` - retains existing functionality;
* `Flush(/*will_reuse_cache=*/false)` - skips destruction and reallocation of the parent cache since it will soon go out of scope anyway;

For the `will_reuse_cache` parameter we want to see exactly which ones will reallocate memory and which won't - since both can be valid usages.

This change was based on a subset of https://github.com/bitcoin/bitcoin/pull/28945.

Co-authored-by: Martin Ankerl <martin.ankerl@gmail.com>

src/test/fuzz/coins_view.cpp

@@ -74,7 +74,7 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsView& backend
                 }
             },
             [&] {
-                (void)coins_view_cache.Flush();
+                (void)coins_view_cache.Flush(/*will_reuse_cache=*/fuzzed_data_provider.ConsumeBool());
             },
             [&] {
                 (void)coins_view_cache.Sync();

src/test/fuzz/coinscache_sim.cpp

@@ -392,7 +392,7 @@ FUZZ_TARGET(coinscache_sim)
                 // Apply to simulation data.
                 flush();
                 // Apply to real caches.
-                caches.back()->Flush();
+                caches.back()->Flush(/*will_reuse_cache=*/provider.ConsumeBool());
             },
 
             [&]() { // Sync.
@@ -402,14 +402,6 @@ FUZZ_TARGET(coinscache_sim)
                 caches.back()->Sync();
             },
 
-            [&]() { // Flush + ReallocateCache.
-                // Apply to simulation data.
-                flush();
-                // Apply to real caches.
-                caches.back()->Flush();
-                caches.back()->ReallocateCache();
-            },
-
             [&]() { // GetCacheSize
                 (void)caches.back()->GetCacheSize();
             },