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Traditionally BTrees in ZODB/py expose point query and iteration APIs.
However they don't allow a BTree to be scanned through concurrently.

For example in wendelin.core each ZBlk1 consists of a IOBTree with 512
chunks

	https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.12-6-g318efce/bigfile/file_zodb.py#L267

and loading those chunks from ZODB one-by-one serially is very slow.

Expose a way to retrieve all children of a B⁺ tree node. This way
loading them all could be started in parallel thus significantly
reducing overall latency if a range or whole BTree needs to be fetched.