Fix 324 : Out of memory errors while updating repos.

Fixes #324 (closed), but in the process makes the updater take a lot longer. My benchmarks tell me that an update which used to take approx 30 seconds on my Nexus 4 now takes about 50-55 seconds. This is because it first inserts the apps into the database (in a temp table) and then subsequently copies that table to the actual table. This means there is a lot more disk access than before.

I'm open for discussion on whether this tradeoff is worth it - however I'll caution that there is always going to be a tradeoff between faster and more memory vs slower and less memory. This is the case with all software, and perhaps more so with memory constrained devices such as phones. Also, as the repo index grows (until perhaps we are able to extract the app descriptions in the future), this will become more of an issue.

I'd also like this to be CR'ed properly before merging, because it changes some important code around the repo updater. It is important because security, and it is also important because it is the main thing that F-Droid needs to do (get a list of apps to show the user).