Look at ways to make slow management commands smaller and quicker

[philgyford]

fetch_flickr_photosets can take a long time (1m 31s for me).

Can we supply an option to say how many things to fetch? And only fetch items that haven't been updated/fetched recently? Might need to add a field to set when they were last updated/fetched?

These also took a long time when they haven't been run for ages, but weren't too bad when run subsequently:

• update_twitter_users (6m 21s for me first time, 45s subsequently)
• fetch_twitter_files (1m 13s first time, 36s subsequently)