Performance callback bug fixes

* Invoke a background error if an error is encountered invoking the performance callback.  We may revisit this as it may be better just to generate an error synchronously from the speedtable search if there is an error in the performance callback.

* Manage reference counts properly when stashing the Tcl result while invoking the performance callback and restoring it afterwards such that we don't crash.

* Fix test to include matching new performance_callback method.