avoid reading from zookeeper when clearing zoocache entries

[keith-turner]

This change indirectly fixes ZombieScanIT. That test was creating scanners running in a thread and interrupting those threads. The interrupts were routinely being ingnored by code in zoocache that ignores interrupts when reading from zookeeper. Looking at the code that was reading from zookeeper, that code was attempting clear entries in the cache. Modified this code to avoid reading from zookeeper when clearing the cache.

[keith-turner]

This change will probably fix a failure in InterruptibleScannersIT also.