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.

ci/test/06_script_b.sh

@@ -46,6 +46,7 @@ if [ "${RUN_TIDY}" = "true" ]; then
           " src/kernel"\
           " src/node/chainstate.cpp"\
           " src/node/mempool_args.cpp"\
+          " src/node/validation_cache_args.cpp"\
           " src/policy/feerate.cpp"\
           " src/policy/packages.cpp"\
           " src/policy/settings.cpp"\