Fix cumulativeUserMemory initial calculation & use double for cumulativeUserMemory consistently

[frankobe]

Currently there is a chance that Cumulative User Memory is much larger than Peak User Memory * Execution Time, refer to the following screenshot of one sample query:

Extra debug logging in TaskContext.java shows,

2021-04-29T18:37:01.352Z    INFO    Task-20210429_183659_00001_stagingva.9.0.16-288 com.facebook.presto.operator.TaskContext    Cumulate user memory: sinceLastPeriodMillis = (34262137327073190 - 0) / 
1_000_000 = 34262137327.073193 ,averageMemoryForLastPeriod= (3184992 + 0) / 2 = 1592496

The bug is that sinceLastPeriodMillis can equal to current system nano time if the lastTaskStatCallNanos is not initialized and defaults to 0.

The fix is basically initialize the lastTaskStatCallNanos to task start time.

Signed-off-by: frankobe mua08p@gmail.com

Test plan - (Please fill in how you tested your changes)

Unit Test

== RELEASE NOTES ==

General Changes
* Fix amplified `Cumulative User Memory` metrics calculation

[frankobe]

@arhimondr Thx 4 review. Would u mind merging it? I am not authorized to do so.