MEDIUM: mworker: set the iocb of the socketpair without using fd_insert()

The worker was previously changing the iocb of the socketpair in the
worker by mworker_accept_wrapper(). However, it was done using
fd_insert() instead of changing directly the callback in the
fdtab[].iocb pointer.

This patch cleans up this by part by removing fd_insert().

It also stops setting tid_bit on the thread mask, the socketpair will be
handled by any thread from now.

src/mworker.c

@@ -425,7 +425,7 @@ static int mworker_pipe_register_per_thread()
 	/* In multi-tread, we need only one thread to process
 	 * events on the pipe with master
 	 */
-	fd_insert(proc_self->ipc_fd[1], fdtab[proc_self->ipc_fd[1]].owner, mworker_accept_wrapper, tid_bit);
+	fdtab[proc_self->ipc_fd[1]].iocb = mworker_accept_wrapper;
 	fd_want_recv(proc_self->ipc_fd[1]);
 	return 1;
 }