Modify fewest_migrations and small_objects ordering strategies

[ppebay]

These have been kept "as-is", with an explicit reference to load (and average load).
They should be re-worked in the more general context of "work".

Related to #167 and #121 but specialized to these 2 specific methods

[ppebay]

This issue deserves either revisiting or closing

[ppebay]

@tlamonthezie we should verify, especially following the latest fixes to transfer strategies in vt (PR 2278/#2201), that those implemented in LBAF are in-sync with those in vt. This will resolve this issue.