cuckoocache: Return approximate memory size

Returning the approximate total size eliminates the need for
InitS*Cache() to do nElems*sizeof(uint256). The cuckoocache has a better
idea of this information.

src/validation.h

@@ -323,7 +323,7 @@ public:
 };
 
 /** Initializes the script-execution cache */
-void InitScriptExecutionCache();
+[[nodiscard]] bool InitScriptExecutionCache();
 
 /** Functions for validating blocks and updating the block tree */