Revert "Evil but Necessary, libapt-pkg not thread safe, afaict."

This reverts commit 362afebc.

Adhoc testing suggested that there is next to no delay in cache opening,
the eventprocessing however essentially causes in-thread-pseudo-threading
which can easily cause crashes if the application does not add access
guards around QApt::Backend. Should this become a problem in the future
again the correct approach would either be to make QApt threadable or
thread aptpkg inside Backend itself and introduce appropriate guarding.

Please note that it is terribly unlikely for blocking open to cause any
problems as muon's initial ::init() is threaded and if an application has
issues with initial open they may as well add access guards and thread
::init() to unblock the UI. Subsequent cache opens will just about always
hit the disk cache and therefore take no time at all.

CCMAIL: echidnaman@kubuntu.org
CCBUG: 330477