wallet: use Mutex for g_sqlite_mutex instead of GlobalMutex

Using `Mutex` provides stronger guarantee than `GlobalMutex` wrt Clang's
thread safety analysis. Thus it is better to reduce the usage of
`GlobalMutex` in favor of `Mutex`.

Using `Mutex` for `g_sqlite_mutex` is ok because its usage is limited in
`wallet/sqlite.cpp` and it does not require propagating the negative
annotations to not relevant code.

src/wallet/sqlite.h

@@ -5,6 +5,7 @@
 #ifndef BITCOIN_WALLET_SQLITE_H
 #define BITCOIN_WALLET_SQLITE_H
 
+#include <sync.h>
 #include <wallet/db.h>
 
 #include <sqlite3.h>
@@ -63,7 +64,16 @@ private:
 
     const std::string m_file_path;
 
-    void Cleanup() noexcept;
+    /**
+     * This mutex protects SQLite initialization and shutdown.
+     * sqlite3_config() and sqlite3_shutdown() are not thread-safe (sqlite3_initialize() is).
+     * Concurrent threads that execute SQLiteDatabase::SQLiteDatabase() should have just one
+     * of them do the init and the rest wait for it to complete before all can proceed.
+     */
+    static Mutex g_sqlite_mutex;
+    static int g_sqlite_count GUARDED_BY(g_sqlite_mutex);
+
+    void Cleanup() noexcept EXCLUSIVE_LOCKS_REQUIRED(!g_sqlite_mutex);
 
 public:
     SQLiteDatabase() = delete;