Solution: when a connection breaks and ZMQ_RECONNECT_IVL is set to -1,
which means a reconnection will not be attempted, send a message from
the I/O thread to the application thread to make the socket call
term_endpoint, which is the equivalent of manually calling
zmq_disconnect.
This way subsequent zmq_connect call to the same endpoint will attempt
again to do a connection.
Otherwise, for some socket types like SUBs, those new connects will
fail as the endpoint is recorded, despite the connection having been
permanently closed.

Add test cases to exercise this corner case with TCP and IPC.