Replace QDateTime::fromString with faster function

This patch to ContactsDatabase::fromDateTimeString led to more than 30% improvement in SeasideCache population time on my device.

P.S. let me know if you want me to squash the two commits. They are two different rewrites of the same function; the later one was a little bit faster.
I'm not certain that every datetime has a fractional part - at one point we had non-fractional times, and they might still be present in extant databases.