applier: stop sending ACKs if master closed socket

If the master closes its end of the socket when there are still unread
rows available for the replica to apply, we will get tons of EPIPE error
messages at the replica's side, emitted every time it attempts to send
an ACK back to the master (i.e. one per each row left in the socket):

  main/107/applierw/ sio.cc:303 !> SystemError writev(2), called on fd 12, aka 127.0.0.1:50852: Broken pipe

To avoid that, let's make the applier writer fiber (the one that sends
ACKs) exit immediately if it receives EPIPE error while trying to send
an ACK.

Closes #2945