Parallelize file transfer for external blobs

[shenshan]

Feature Request

Problem

Fetching from an internal blob field and from external blob fields have big difference in performance.

Same to this issue on the python site: https://github.com/datajoint/datajoint-python/issues/806

Requirements

We should consider parallelizing the file downloads from external storage.

Benchmark

Here is a benchmark plotting showing the performance difference. This is for princeton database, running on a computer in the lab. The external storage on a shared drive in the institute.

[shenshan]