Write only permission for fields

[gilbo]

There are some tricky issues about the write-only permission

[schinmayee]

We have the bare minimum needed for multiple partitions now. The following issues still exist:

• "Write only" is not handled right now -- it is safe to use "write only" if data is unconditionally written for all rows.
• We do not use reduce features of Legion for multiple partitions -- when we do start using it, we should record reduce as a reduce privilege instead of write.