[enhancement] Various enhancements to the results DB feature

[vkarak]

More specifically, this PR adds the following enhancements:

• Support for computed virtual test attributes. This mechanism allows us to extend the test queries in existing databases without having to migrate the DBs and update the test records.
  • basename: the test's name without any parameters (essentially the test class name). This is useful for parameter-based aggregations.
  • sysenv: the formatted system, partition, environment combination
  • pdiff: the performance difference when doing comparisons
• Support for completely custom groupings. The syntax os the CMPSPEC is extended to get an explicit list of attributes for grouping test cases.
• Support for arbitrary table column selection. The syntax of the CMPSPEC is extended to get an explicit list of attribute columns to show. As a result the --table-hide-columns option is removed as now the exact columns of the output can be specified.
• Extend --list-stored-testcases to show the performance summary of the selected cases. It uses the same CMPSPEC syntax.
• Format parameters when storing a report/session using the parameter's format function.
• Show A/B absolute values for non-aggregated attributes/columns.
• Enable -E for testcase filtering.
• Session extras keys are now stored unchanged in the DB (previously, they were prefixed with $). This allows them to be properly filtered with the session filtering expressions passed to --session-filter.
• The --session-extras option can now be specified multiple times.
• Replace pretty table format with outline and add a new grid format. This is to allow nice printing of multiline cells.
• Remove --session-filter option and replace it with special syntax in the CMPSPEC. This allows us to select and compare different sets of sessions.
• Clean up and homogenize the internal storage API.
• All options dealing with sessions (e.g., --list-stored-sessions, --delete-sessions) are now accepting the extended query syntax (over time period, session uuid or session filter).
• Options that used to manipulate a single session have now their names converted to plural.

Docs and unit tests are updated accordingly.

[vkarak]

As another improvement, what would you think of making --session-extras an "append" option so that it can be specified multiple times to augment any "default" extras passed for example in a pipeline?