[DBTablesMonitoring] Use a defined type instead of a raw type.
[mlpl][SmartTime] Add a new method: hasValidTime().
[DBTablesTest] Use a defined type instead of a raw type.
[DBTablesTest] Use a defined type instead of a raw type.
[DBTablesTest] Place test events in ascending order of time.

Normally events are saved in order of the time.
[DBTablesMonitoring] Add a new method: getTimeOfLastEvent().

Getting monitoring data from OpenStack's ceilometer is done
by each trigger. So we needs the last event information by
triggers.
Merge pull request #539 from project-hatohol/dbmonitoring-get-last-event-time

Dbmonitoring get last event time