fix a bug in Trace.dryrun that cause the last Action not being exported.
PageRowRDD.fetch now support lookup from pages of itself and other RDD (through lookupFrom parameter). dryrun is used to find those pages. Dryrun should be used in DFS cache as well to reduce code complexity and to allow more efficient use of refreshed pages. MORE INTEGRATION TEST NEEDED
move some tests on action to the right file
add FetchIT to integration test