NEXUS-7671: Purge causes OOM on large TL

Change to purge in batches, to conserve heap memory
NEXUS-7671: Remove unneeded maybeRefresh
NEXUS-7671: Remove irrelevant UTs

That were testing feature purge-by-type/by-subtype and it's combinations
that were never actually used in production. Timeline interface does not
support this, only the indexer and it was tested only in the indexer
UTs, nowhere else.

It's broken as deletion happens per-term, and term includes timestamp only
but not the type nor subtype.
Merge remote-tracking branch 'origin/nexus-2.11.x' into nexus-2.11.x-NEXUS-7671-purge-oom
NEXUS-7671: Applied comments and some doco
NEXUS-7671: Remove ignored tests and resources

as NX3 has completely rewritten TL backed with OrientDB
Merge pull request #861 from sonatype/nexus-2.11.x-NEXUS-7671-purge-oom

NEXUS-7671: Purge causes OOM on large TL