kernel: De-globalize script execution cache

Move its ownership to the ChainstateManager class.

Next to simplifying usage of the kernel library by no longer requiring
manual setup of the cache prior to using validation code, it also slims
down the amount of memory allocated by BasicTestingSetup.

src/script/sigcache.h

@@ -16,6 +16,7 @@
 // systems). Due to how we count cache size, actual memory usage is slightly
 // more (~32.25 MiB)
 static constexpr size_t DEFAULT_MAX_SIG_CACHE_BYTES{32 << 20};
+static constexpr size_t DEFAULT_SCRIPT_EXECUTION_CACHE_BYTES{DEFAULT_MAX_SIG_CACHE_BYTES / 2};
 
 class CPubKey;