Implement support for multi-threaded validation

[qligier]

Implement support for multi-threaded validation

When SchematronixValidator processes the Schematron rules, it reads and executes them one by one. This could certainly be improved by dispatching SchematronixRules to a fixed number of threads (with a ThreadPoolExecutor?) for execution.

Open questions:

• Thread safety: could multiple Saxon Processors work at the same time from different threads on the same DOM?
• Memory optimization: is it possible for the validator to wait for a free thread before parsing a new rule, instead of parsing the whole Schematron and dispatching them all? Or maybe keeping a single rule in
• Fast validation: is it possible to stop all threads if a single one reports a validation error while on fast validation mode?