tcpip_callback_wait() function

Sends a message to TCPIP thread to call a function. Caller thread blocks until the function returns. It is recommended to use LWIP_TCPIP_CORE_LOCKING (preferred) or LWIP_NETCONN_SEM_PER_THREAD. If not, a semaphore is created and destroyed on every call which is usually an expensive/slow operation.