Improved the incremental sending of file lists in two ways: (1)

when the sender gets an index of a file to send, we make sure to
send enough future file-list data relative to the new cur_flist
value before we send the data for the file transfer (ensuring that
the generator gets the data in a more timely manner), and (2) the
generator flushes the output socket after each file-list object
has completed its scan so that the sender knows in a more timely
manner that more list data is needed.