The file lookup is the single most expensive operation when loading
events from a profiled application, in particular because so far we're
doing one lookup per source location, many of which are actually in the
same files. Caching those lookups for a short time dramatically
increases performance.

Change-Id: Iaa47327aca3f34cbad194757d3ba152205788682
Reviewed-by: Kai Koehne <kai.koehne@digia.com>