Switch listener to same incoming command handling as sender.

Rather than using a thread-safe queue, threads sending messages to
listeners atomically reserve a message struct, write a command into
it, and commit the command by writing the message ID into the listener
thread's incoming command pipe. It polls on network IO and the command
pipe, so commands wake it up. It reads the ID, processes the command
written into l->msgs[ID], and atomically releases the message struct.