Patch for SPARK-942

This patch is intended to fix the issues described in https://spark-project.atlassian.net/browse/SPARK-942

Essentially if a user returns a generative iterator (from a flatMap operation), when trying to persist the data, Spark would first unroll the iterator into an ArrayBuffer, and then try to figure out if it could store the data. In cases where the user provided an iterator that generated more data then available memory, this would case a crash. With this patch, if the user requests a persist with a 'StorageLevel.DISK_ONLY', the iterator will be unrolled as it is inputed into the serializer. 

To do this, two changes where made:
1) The type of the 'values' argument in the putValues method of the BlockStore interface was changed from ArrayBuffer to Iterator (and all code interfacing with this method was modified to connect correctly.
2) The JavaSerializer now calls the ObjectOutputStream 'reset' method every 1000 objects. This was done because the ObjectOutputStream caches objects (thus preventing them from being GC'd) to write more compact serialization. If reset is never called, eventually the memory fills up, if it is called too often then the serialization streams become much larger because of redundant class descriptions.