aptirepo-updatedistsd: add new daemon for updating dists on-demand

It can be used to separate importing and updatedists from each other.
Updatedists is quite time-consuming operation but importing is
relatively fast, it is basically just copying files into pool.

Currently aptirepo-http updates dists after every package import, which
makes importing quite slow becase updatedists is slow and aptirepo is
locked for the duration of import + updatedists. This daemon is designed
to remove the burden of dist updating from the http-server by running
update-dists periodically behind the scenes. Server just needs to inform
the updatedists daemon of repositories requiring update.
aptirepo-updatedistsd: add missing import
aptirepo-updatedistsd: make aptirepo-updatedistsd log something useful
aptirepo-updatedistsd: improve aptirepo kwarg handling
aptirepo-updatedistsd: add arg parsing
aptirepo-updatedistsd: ensure socket file is removed on exit
aptirepo-updatedistsd: change the default socket filepath
aptirepo-updatedistsd: remove repo from queue on error
aptirepo-updatedistsd: increase the min interval between updates to 60 secs
aptirepo-updatedistsd: make select() sleep max 5 secs

It makes the daemon more responsive.
aptirepo-updatedistsd: improve update start log message
aptirepo-updatedistsd: remove unused import