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/script/sigcache.cpp

@@ -75,7 +75,7 @@ public:
         std::unique_lock<std::shared_mutex> lock(cs_sigcache);
         setValid.insert(entry);
     }
-    uint32_t setup_bytes(size_t n)
+    std::pair<uint32_t, size_t> setup_bytes(size_t n)
     {
         return setValid.setup_bytes(n);
     }
@@ -92,14 +92,18 @@ static CSignatureCache signatureCache;
 
 // To be called once in AppInitMain/BasicTestingSetup to initialize the
 // signatureCache.
-void InitSignatureCache()
+bool InitSignatureCache()
 {
     // nMaxCacheSize is unsigned. If -maxsigcachesize is set to zero,
     // setup_bytes creates the minimum possible cache (2 elements).
     size_t nMaxCacheSize = std::min(std::max((int64_t)0, gArgs.GetIntArg("-maxsigcachesize", DEFAULT_MAX_SIG_CACHE_SIZE) / 2), MAX_MAX_SIG_CACHE_SIZE) * ((size_t) 1 << 20);
-    size_t nElems = signatureCache.setup_bytes(nMaxCacheSize);
+
+    auto setup_results = signatureCache.setup_bytes(nMaxCacheSize);
+
+    const auto [num_elems, approx_size_bytes] = setup_results;
     LogPrintf("Using %zu MiB out of %zu/2 requested for signature cache, able to store %zu elements\n",
-            (nElems*sizeof(uint256)) >>20, (nMaxCacheSize*2)>>20, nElems);
+              approx_size_bytes >> 20, (nMaxCacheSize * 2) >> 20, num_elems);
+    return true;
 }
 
 bool CachingTransactionSignatureChecker::VerifyECDSASignature(const std::vector<unsigned char>& vchSig, const CPubKey& pubkey, const uint256& sighash) const