New scheme for keeping the per-process tcl_precision value in sync without
the need for mutex locks on every read.  Uses adapted ProcessGlobalValue
machinery backported from Tcl 8.5 where it's been working without reported
problems.  Thanks to Phil Brooks for reporting on tests which highlight the
thread performance problems raised by the old scheme, and to Clif Flynt for
further testing pointing the finger at tcl_precision locks as the main culprit.