Manage the interruption in ZKUtil#getData

Many, but not all, methods in ZKUtil partly hides the interruption: they return null or something like that. Many times, this will result in a NPE, or something undefined.

This jira is limited to the getData to keep things small enough (it's used in hbase-client code).
The code is supposed to behave at least 'as well as before', or better (hopefully).

It could be included in a later .98 release (98.1) and in .99