ENG-6036 timestamp value as strings
generate sql once
only call generator once, and optimize string Concatenation
clean SQLCoverageReport.py, remove debug comment
clean debug comment in sql_coverage_test.py
clean code for sql_coverage
Merge pull request #1376 from VoltDB/ENG-6036-simplify_SQLGenerator

ENG-6036 generate timestamps as strings and generate report faster
Merge branch 'master' of github.com:VoltDB/voltdb into ENG-6259-subselect-final