Reinstates a batching page cache for batch import

Basically it's the "page cache" version of
https://github.com/neo4j/neo4j/blob/2.1.2/community/kernel/src/main/java/org/neo4j/unsafe/impl/batchimport/store/BatchFriendlyWindowPoolFactory.java

such that there's only one page per store in memory all the time and it
moves along with the batch importer "cursor", no other store file memory required.
Merge pull request #2640 from tinwelint/2.2-batch-page-cache

Reinstates a batching page cache for batch import