validationcaches: Add and use ValidationCacheSizes

Also:

- Make DEFAULT_MAX_SIG_CACHE_SIZE into constexpr
  DEFAULT_MAX_SIG_CACHE_BYTES to utilize the compile-time integer
  arithmetic overflow checking available to constexpr.
- Fix comment (MiB instead of MB) for DEFAULT_MAX_SIG_CACHE_BYTES.
- Pass in max_size_bytes parameter to InitS*Cache(), modify log line to
  no longer allude to maxsigcachesize being split evenly between the two
  validation caches.
- Fix possible integer truncation and add a comment.

[META] I've kept the integer types as int64_t in order to not introduce
       unintended behaviour changes, in the next commit we will make
       them size_t.

src/validation.cpp

@@ -1656,7 +1656,8 @@ bool CScriptCheck::operator()() {
 static CuckooCache::cache<uint256, SignatureCacheHasher> g_scriptExecutionCache;
 static CSHA256 g_scriptExecutionCacheHasher;
 
-bool InitScriptExecutionCache() {
+bool InitScriptExecutionCache(int64_t max_size_bytes)
+{
     // Setup the salted hasher
     uint256 nonce = GetRandHash();
     // We want the nonce to be 64 bytes long to force the hasher to process
@@ -1666,14 +1667,14 @@ bool InitScriptExecutionCache() {
     g_scriptExecutionCacheHasher.Write(nonce.begin(), 32);
     // nMaxCacheSize is unsigned. If -maxsigcachesize is set to zero,
     // setup_bytes creates the minimum possible cache (2 elements).
-    size_t nMaxCacheSize = std::max((int64_t)0, gArgs.GetIntArg("-maxsigcachesize", DEFAULT_MAX_SIG_CACHE_SIZE) / 2) * ((size_t) 1 << 20);
+    size_t nMaxCacheSize = std::max<int64_t>(max_size_bytes, 0);
 
     auto setup_results = g_scriptExecutionCache.setup_bytes(nMaxCacheSize);
     if (!setup_results) return false;
 
     const auto [num_elems, approx_size_bytes] = *setup_results;
-    LogPrintf("Using %zu MiB out of %zu/2 requested for script execution cache, able to store %zu elements\n",
-              approx_size_bytes >> 20, (nMaxCacheSize * 2) >> 20, num_elems);
+    LogPrintf("Using %zu MiB out of %zu MiB requested for script execution cache, able to store %zu elements\n",
+              approx_size_bytes >> 20, max_size_bytes >> 20, num_elems);
     return true;
 }