sockets are not thread-safe

[tabascoeye]

When a socket is used from different tasks at the same time, the stack behavior is basically undefined.

To slowly work towards the goal of having a thread-safe stack that is still "light weight", we'll try to do the following bits:

• first allow another Task to close a socket that is blocking in accept(), connect() or select()
• analyze what would be necessary to make it fully independent:
  • the api_message interface is waiting for a op_completed semaphore and it cannot be determined what operation has completed when multiple ones where started
  • check for possible raceb conditions in the stack

[tabascoeye]

found another possible race condition when using sockets from multiple thread contexts: the setsockopt calls are handled via a tcpip_callback() so it can happen that the stack only processes them when the socket itself is already closed. Then, inside the lwip_setsockopt_internal() function, writes to NULL pointers are made on sock->conn->pcb.ip