IMPALA-5599 broke a client software due to precision change in date-time string

IMPALA-5599 changed the precision of Start Time and End Time date-time strings of query profiles to microseconds. This ended up breaking compatibility with a management client software.

This JIRA is being raised to restore the precision to what it was (nanoseconds) , to address the breakage. Longer term, the interface contract between Impala and such client software will be made stricter and more efficient, and avoid having clients parse what are meant to be human-readable information.

The Impala testing also needs to be enhanced to ensure interface contracts are not broken.