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/bitcoin-chainstate.cpp

@@ -13,6 +13,7 @@
 
 #include <kernel/checks.h>
 #include <kernel/context.h>
+#include <kernel/validation_cache_sizes.h>
 
 #include <chainparams.h>
 #include <consensus/validation.h>
@@ -62,8 +63,9 @@ 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.
-    Assert(InitSignatureCache());
-    Assert(InitScriptExecutionCache());
+    kernel::ValidationCacheSizes validation_cache_sizes{};
+    Assert(InitSignatureCache(validation_cache_sizes.signature_cache_bytes));
+    Assert(InitScriptExecutionCache(validation_cache_sizes.script_execution_cache_bytes));
 
 
     // SETUP: Scheduling and Background Signals