Consider: Allow multi-key requests to receive their data callback in order

[ecton]

The current implementation of get_multiple and scan both take a key reader and key evaluator function. The reader is invoked for each key as it is found, and the result controls the iteration over the tree. If data is requested to be read, it will be requested after the scan is completed, ordered from the start of the file to the end of the file to try to optimize access.

For callers that need to scan the content of the data but might still want to abort the scan, this forces the scan to iterate over the entire tree. This change in functionality could enable the key evaluator to be invoked immediately after the read data request is returned. This would allow the evaluator or next call to the reader to stop the scan.

Currently, no code that calls these functions in BonsaiDb needs this functionality, but as a general purpose library, this functionality could be very useful.