Fixes #954

The problem was that the clear method triggers on all partitions a RecordStore to be created,
even if there are no record. This fix doesn't trigger the created of the RecordStore.
ClearOperation.shouldBackup is restored to true (was put to false to locate bug)
Minor improvement; if the recordstore is null, then there is no need to create a backup
Fixed issue with the test
Merge pull request #1106 from pveentjer/fix/3.x/954-map-clear-memory-problem

Fixes #954