Restructure exitlists module

A smaller module.

ExitList processing:

• 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).