[Predator] Cache the crash_iterator for delta test.

Design doc: https://docs.google.com/a/google.com/document/d/1kfu_HIJFCSwOWQleRpSkOH4t_D0UFY5vDSSausaZuqs/edit?usp=sharing#heading=h.n5y4m5llnog3

For Delta test on app engine, since crash_iterator will download crash infos to local disk and use them as local cache. For subprocess run-findit, it can read crash infos from those local cache files instead of getting those data from PIPE.

This can speed up the delta test and avoid potential issue of PIPE big chunk of data to run-predator process.