NEXUS-7671: Purge causes OOM on large TL

Change to purge in batches, to conserve heap memory.

PR Tested with RSO TL data (11GB). Purge done in around 6 mins reducing TL from 11GB to 680MB. Heap monitored, did not get higher than 300-350 MB (NX 2.11-SNAPSHOT Pro with default mem settings).

Issue
https://issues.sonatype.org/browse/NEXUS-7671

CI
http://bamboo.s/browse/NX-OSSF387
do we plan on un-ignoring these in the future?  else throw them away