Get rid of warning messages (1) "could not write super block; waiting
10 seconds" and (2) "blistAlloc: called on clean block".

(1) Add a waitlock parameter to blockWrite, so superWrite can tell it
to wait when it finds the dependent root block locked when trying to
write out the new epoch's super block.

(2) Don't try to allocate an unneeded BList to remove the old epoch's
root block from the active file system, in the rare case when the
super block has already been written out.