flush() is redundant, it makes more sense to just write the file on close().
There is no reason why you want to commit the current state of the cache to disk
at any other time then when closing the application.
A new datastructure is about to take dlhist place. dlhist is currently
implemented as a mixture of an "process cache" that should record what
rss items has been processed (that is why the url is used as a unique
identifier), but right now it only stores an url if it has been
downloaded. A new datastructure that should be "download history"
shall be implemented, that will keep track of what title and where
it has been downloaded to. this will make it possible to only
download an rss title to a location once.
Splitting this datastructure into two separated structures is trivial
as a "process cache" will threat URL's as a unique identifier and
a "download history" will threat the title in an rss item as a
unique identifier (and also track it's destinations).
This commit does not change any functionality, I just rename
this to keep the "dlhist" prefix and source files clear for
when implementing the real dlhist.
