HPCC-8828 Add new metrics to Roxie

Various minor bug fixes and performance enhancements in Roxie metrics
reporting.

Add some global metrics to supplement the ones taht are reported per query or
per query class.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-8828 Add new metrics to Roxie

Some changes in response to code review. Use accessor functions rather than
global variables.

Also looked a little closer at which metrics it is sensible to reset.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4064 from richardkchapman/roxie-stats

HPCC-8828 Add new metrics to Roxie

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>