perf optimization and interface simplification

@dmhood 

* no longer initializing meta in constructor, since scanning records to build meta is only needed when we need to purge
* constructor is back to synchronous
* use async.mapSeries to guarantee purge order
* eliminates recursive call to purge()
* remove scanSize option. just use purgeLoadIncrase, for API simplicity
Unless you want to keep this for readability we don't really need this object anymore.