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/node/chainstatemanager_args.cpp

@@ -63,6 +63,7 @@ util::Result<void> ApplyArgsManOptions(const ArgsManager& args, ChainstateManage
         // 2. Multiply first, divide after to avoid integer truncation.
         size_t clamped_size_each = std::max<int64_t>(*max_size, 0) * (1 << 20) / 2;
         opts.script_execution_cache_bytes = clamped_size_each;
+        opts.signature_cache_bytes = clamped_size_each;
     }
 
     return {};