Restructure exitlists module
A smaller module.
- download (see ticket legacy/trac#20516 (moved) for replacing httpurlconnection)
- parses data using metrics-lib
- verifies age of most recent measurement
- looks for the last three lists received (What is this statistics for?)
- performs a clean-up in recent-folder by cut-off time.
These five steps could be structured a little. The cleaning step should be moved to a general place (cf. note in parent ticket).