TODO hardlink performance optimizations

[Wayne Davison]

One complicating factor of getting rid of dev+inode in the flist_struct
is that some of the old-protocol code uses these values to fix a bug in
directory deletion order in protocols prior to 19.  I'd suggest that we
just dump support for protocols 17 and 18 in the next release.  Protocol
19 is almost 6 years old, and our current code even warns the user when
using a protocol any older than 20, so I doubt that this would cause a
problem.

..wayne..