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/bitcoin-chainstate.cpp

@@ -62,8 +62,8 @@ int main(int argc, char* argv[])
     // Necessary for CheckInputScripts (eventually called by ProcessNewBlock),
     // which will try the script cache first and fall back to actually
     // performing the check with the signature cache.
-    InitSignatureCache();
-    InitScriptExecutionCache();
+    Assert(InitSignatureCache());
+    Assert(InitScriptExecutionCache());
 
 
     // SETUP: Scheduling and Background Signals