Variable query support for HistoricProcessInstances

The ProcessInstanceQuery supports querying for running process instances the process context of which contains certain variables (using the variableValue* methods).

If the history level is set to FULL, historic process context variables are also available, but they currently cannot be used to restrict the result set produced by the HistoricProcessInstanceQuery.

I am aware that this may not be a trivial task given the fact that there may be multiple historic versions of a process context variable and that this may be not very beneficial to the performance, but for forensic/auditing purposes, such added functionality would still be very helpful.