validation: reuse same CCoinsViewCache for every ConnectBlock call

Add m_connect_block_view to ChainState's CoinsViews.
Call CreateResetGuard inside ConnectTip to ensure the view
is Reset after each block, avoiding repeated memory allocations.

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

src/validation.h

@@ -488,6 +488,10 @@ public:
     //! can fit per the dbcache setting.
     std::unique_ptr<CCoinsViewCache> m_cacheview GUARDED_BY(cs_main);
 
+    //! Temporary CCoinsViewCache layered on top of m_cacheview and passed to ConnectBlock().
+    //! Reset between calls and flushed only on success, so invalid blocks don't pollute the underlying cache.
+    std::unique_ptr<CCoinsViewCache> m_connect_block_view GUARDED_BY(cs_main);
+
     //! This constructor initializes CCoinsViewDB and CCoinsViewErrorCatcher instances, but it
     //! *does not* create a CCoinsViewCache instance by default. This is done separately because the
     //! presence of the cache has implications on whether or not we're allowed to flush the cache's