- Renamed log_error_fd to msg_fd_out, and io_error_fd to msg_fd_in.
- Added a redo_list* to keep track of any redo items that come over
  the message channel from the receiver to the generator.
- The io_flush() and msg_list_push() functions now take an arg:
  NORMAL_FLUSH or FULL_FLUSH.  The latter ensures that we don't lose
  any log messages that haven't gotten written from the msg_list.
- Added a send_msg() function that log.c calls to send any remote
  log messages.  It is also called to send the new MSG_REDO and
  MSG_DONE messages.