Optimization in KDE-PIM
Use QCryptographicHash instead of KMd5 and don't base64 encode the MD5 hash.
QCryptographicHash is about 2x faster for the input sizes we have here
(see benchmark in kdecore). The base64 encoding was totally pointless,
wasting 24 byte per message item (when threading is enabled) and slowing
down comparison/hashing by about 30%.