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.h

@@ -323,7 +323,7 @@ public:
 };
 
 /** Initializes the script-execution cache */
-[[nodiscard]] bool InitScriptExecutionCache();
+[[nodiscard]] bool InitScriptExecutionCache(int64_t max_size_bytes);
 
 /** Functions for validating blocks and updating the block tree */