NIFI-5339 - Better Time Tracking for ExecuteSQL Durations

[patricker]

ExecuteSQL uses an attribute, executesql.query.duration, to track how long query execution and fetch took.

For multiple result set queries this is a cumulative/running total, and is not specific to each query.

Also, there is no separation between query execution and fetch times.

Updates:

Add a dedicated property executesql.query.executiontime to track the base query execution time.
Add a dedicated property executesql.query.fetchtime to track the per resultset fetch time.
Update the existing executesql.query.duration to be the sum of the other two times.

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[pvillard31]

+1, code LGTM, merging to master, thanks @patricker