hbaseindex: allow to set scanner cache size

[bruno]

Querying hbase indexes can probably be sped up by setting a cache size larger than 1 for the scanners.

Need to think about whether we can use a reasonable default (like 100 or so). Or maybe this should be an per-index setting.

This can also be set globally using hbase.client.scanner.caching

[bruno]

This has been changed to use a default scanner size of 30. Making this configurable on the level of index and individual query still to do. Unassigning from 0.3.