client: Create clean-cache flag during client commit
client: Create clean-cache flag during client commit

- Add touch() helper function to create an empty file.  Update functions
to use it.

- Create the clean-cache flag during client commit, so that repeated
"isr sync" operations will not repeatedly create new versions

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr