kernel: Add chainstate manager option for setting worker threads

Re-use the same pattern used for the context options. This allows users
to set the number of threads used in the validation thread pool.

src/test/kernel/test_kernel.cpp

@@ -502,5 +502,6 @@ BOOST_AUTO_TEST_CASE(btck_chainman_tests)
     auto context{create_context(notifications, ChainType::MAINNET)};
 
     ChainstateManagerOptions chainman_opts{context, test_directory.m_directory.string(), (test_directory.m_directory / "blocks").string()};
+    chainman_opts.SetWorkerThreads(4);
     ChainMan chainman{context, chainman_opts};
 }