kernel: De-globalize signature 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.

Use this opportunity to make SignatureCache RAII styled

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/bitcoin-chainstate.cpp

@@ -15,7 +15,6 @@
 #include <kernel/chainstatemanager_opts.h>
 #include <kernel/checks.h>
 #include <kernel/context.h>
-#include <kernel/validation_cache_sizes.h>
 #include <kernel/warning.h>
 
 #include <consensus/validation.h>
@@ -63,12 +62,6 @@ int main(int argc, char* argv[])
     // properly
     assert(kernel::SanityChecks(kernel_context));
 
-    // 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.
-    kernel::ValidationCacheSizes validation_cache_sizes{};
-    Assert(InitSignatureCache(validation_cache_sizes.signature_cache_bytes));
-
     ValidationSignals validation_signals{std::make_unique<util::ImmediateTaskRunner>()};
 
     class KernelNotifications : public kernel::Notifications