Store time in int64 type instead of time.Time

In various places in Yunikorn, we store time like:

Application:

SubmissionTime       time.Time
finishedTime         time.Time 
startTime            time.Time 

AllocationLogEntry:

LastOccurrence      time.Time

AllocationAsk:

createTime          time.Time 

In the DAO objects, we convert it back to int64 with Time.UnixNano(). There are calculations at various places (eg. Since(), Sub()), but that's simple to solve.

Storing a simple int64 is better because not only it consumes less space, time.Time also contains a pointer to a Location structure which we don't use.