diamondd: Put filter code in a search-specific temporary directory
diamondd: Put filter code in a search-specific temporary directory

Copy, or link if possible, filter code from the blob cache into a
search-specific temporary directory before running it.  When blob cache
garbage collection is added, this will ensure that filters are not
garbage-collected while they are running.

cmusatyalab/opendiamond,cmusatyalab/opendiamond,cmusatyalab/opendiamond,cmusatyalab/opendiamond,cmusatyalab/opendiamond