Branch b1_4

Branch b1_4
It was possible to get multiple mfd references during close and client
eviction, leading to either mds_close() or mds_mfd_close() referencing
a freed mfd.

Now we remove the mfd from the mfd_list and the handle hash under lock so
that once it starts on the road to destruction it is not possible to get a
new reference to it.  Also add comments on usage of mfd-related functions!
b=3819, b=4364, b=4397, b=6313
r=green