refactor: Use Mutex type for g_cs_recent_confirmed_transactions
refactor: Use Mutex type for g_cs_recent_confirmed_transactions

Former-commit-id: 8fe5e4d6d4f246dcbd6047b58c7ef2068d77e8cc
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin