Merge bitcoin/bitcoin#33866: refactor: Let CCoinsViewCache::BatchWrite return void

6da6f503a6 refactor: Let CCoinsViewCache::BatchWrite return void (TheCharlatan)

Pull request description:

  CCoinsViewCache::BatchWrite always returns true if called from a backed cache, so just return void instead. Also return void from ::Sync and ::Flush.

  This allows for dropping a FatalError condition and simplifying some dead error handling code a bit.

  Since we now no longer exercise the "error path" when returning from `CCoinsView::BatchWrite`, make the method clear the cache instead. This should only be exercised by tests and not change production behaviour. This might slightly improve the coins_view fuzz test's ability to generate better coverage.

ACKs for top commit:
  l0rinc:
    ACK 6da6f503a6
  andrewtoth:
    re-ACK 6da6f503a6
  achow101:
    ACK 6da6f503a6
  w0xlt:
    ACK 6da6f503a6

Tree-SHA512: dfaa325b0cf8108910aebf1b27434aaddb639d10d860e96797c77ea42eca9035a54a7dc1d6a5d4eae2b75fcc9356206d3d5672243d2c906e80d19024c8b95408

src/test/fuzz/coinscache_sim.cpp

@@ -163,7 +163,7 @@ public:
     std::unique_ptr<CCoinsViewCursor> Cursor() const final { return {}; }
     size_t EstimateSize() const final { return m_data.size(); }
 
-    bool BatchWrite(CoinsViewCacheCursor& cursor, const uint256&) final
+    void BatchWrite(CoinsViewCacheCursor& cursor, const uint256&) final
     {
         for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)) {
             if (it->second.IsDirty()) {
@@ -187,7 +187,6 @@ public:
                 }
             }
         }
-        return true;
     }
 };