* Do HTTP head request first when loading database from mirror to avoid
downloading the full database all the time
* Use the last modification date of the local database file because with
the persistent storage even local database reloads became a bit expensive
Usually this doesn't make sense because we want to build all packages which
were determined in the previous prepare action. When specifying the
packages again in the conduct build action we'd skip packages which are
pulled into the build as dependencies which is normally not desired.
* Prevent endless recursion
* Consider starting parallel sequence also as having started an action to
prevent starting also actions which should only start after that parallel
sequence
