Search Caching possibly broken

[Paul-Ferrell]

Pavilion is supposed to be keeping a picked dictionary as a cache for both series and test_runs. This dictionary allows us to skip the expensive FS reads needed to gather the information on a test_run/series, provided that the test_run/series has completed.

This functionality seems to be broken. Check whether that's actually the case, and fix it. This is defined in the dir_db.py module.