Convert event timestamp to PrecisionTimestamp

Before the timestamps for event and trait are stored in a Float
In mysql, the default precision won't work well, this one are rounded.

This patch, change the Float by the PrecisionTimestamp Column Type, like
the sample timestamp that demand the same precision.

And ensure that get_events always return datetime object.

Change-Id: I1f986784b07afa50bdbb7faa9499e7b614da8ec1
Merge "Convert event timestamp to PrecisionTimestamp"