Restore Atime weirdness

[Sebastian Spaeth]

Hi all,

while working on an initial --dry-run function I had a look at our
"restore atime" code for the very first time.

Please find attached patches, that simplify and document the code
without functional changes.

One question: Does ANYONE actually use it and what is the purpose :-)? I
understand what it does, but not why someone would be needing it...

#1 Move testing of stuff out of BaseRepository and into MaildirRepositoy
#2 Some code beautification
#3 User documentation improvement and Changelog entry

Sebastian