refactor: add thread safety lock assertion to WriteBlockIndexDB()

The new helper function, BlockManager::WriteBlockIndexDB(),
has a thread safety lock annotation in its declaration but is
missing the corresponding run-time lock assertion in its definition.

Per doc/developer-notes.md: "Combine annotations in function
declarations with run-time asserts in function definitions."

src/node/blockstorage.cpp

@@ -340,6 +340,7 @@ void BlockManager::Unload()
 
 bool BlockManager::WriteBlockIndexDB()
 {
+    AssertLockHeld(::cs_main);
     std::vector<std::pair<int, const CBlockFileInfo*>> vFiles;
     vFiles.reserve(m_dirty_fileinfo.size());
     for (std::set<int>::iterator it = m_dirty_fileinfo.begin(); it != m_dirty_fileinfo.end();) {