Use the TreeWatcher interface in pathfs to synchronize the parent
mapping in pathfs with the tree management in nodefs.

Tested:
  loopback -l $mount $orig
  fsstress -p 1 -z -f rmdir=10 -f link=10 -f creat=10 -f mkdir=10 \
    -f rename=30 -f stat=30 -f unlink=30 -f truncate=20 -m 8 -n 1000 \
    -d $mount -s 1456162204 ; sleep 1; echo ok ; rm -Rf $mount

Fixes #99.

Change-Id: I8668571c7dabf6da6ec708eec53de1887864b9ea