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/kernel/bitcoinkernel.cpp

@@ -689,6 +689,12 @@ btck_ChainstateManagerOptions* btck_chainstate_manager_options_create(const btck
     }
 }
 
+void btck_chainstate_manager_options_set_worker_threads_num(btck_ChainstateManagerOptions* opts, int worker_threads)
+{
+    LOCK(btck_ChainstateManagerOptions::get(opts).m_mutex);
+    btck_ChainstateManagerOptions::get(opts).m_chainman_options.worker_threads_num = worker_threads;
+}
+
 void btck_chainstate_manager_options_destroy(btck_ChainstateManagerOptions* options)
 {
     delete options;

src/kernel/bitcoinkernel.h

@@ -729,6 +729,18 @@ BITCOINKERNEL_API btck_ChainstateManagerOptions* BITCOINKERNEL_WARN_UNUSED_RESUL
     const char* blocks_directory,
     size_t blocks_directory_len) BITCOINKERNEL_ARG_NONNULL(1, 2);
 
+/**
+ * @brief Set the number of available worker threads used during validation.
+ *
+ * @param[in] chainstate_manager_options Non-null, options to be set.
+ * @param[in] worker_threads             The number of worker threads that should be spawned in the thread pool
+ *                                       used for validation. When set to 0 no parallel verification is done.
+ *                                       The value range is clamped internally between 0 and 15.
+ */
+BITCOINKERNEL_API void btck_chainstate_manager_options_set_worker_threads_num(
+    btck_ChainstateManagerOptions* chainstate_manager_options,
+    int worker_threads) BITCOINKERNEL_ARG_NONNULL(1);
+
 /**
  * Destroy the chainstate manager options.
  */

src/kernel/bitcoinkernel_wrapper.h

@@ -628,6 +628,11 @@ public:
         : UniqueHandle{btck_chainstate_manager_options_create(context.get(), data_dir.c_str(), data_dir.length(), blocks_dir.c_str(), blocks_dir.length())}
     {
     }
+
+    void SetWorkerThreads(int worker_threads)
+    {
+        btck_chainstate_manager_options_set_worker_threads_num(get(), worker_threads);
+    }
 };
 
 class ChainMan : UniqueHandle<btck_ChainstateManager, btck_chainstate_manager_destroy>

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};
 }