interleave::get is "expensive"

Part of troubleshooting of
- #1549

running a straightforward download

```
rm -rf sub-*; DANDI_DEVEL_AGGRESSIVE_RETRY=1 dandi --log-level=INFO download -e refresh -J 1:1 https://api.dandiarchive.org/api/assets/f0da6521-b9c7-4fa5-9405-c48527aebe90/download/
```
which is a single file/thread download leads to dandi CLI being 10-20% CPU and py-spy showing


so sampling profiler finds itself most often in `get (interleave/__init__.py)`. There are multiple `get`s in that file but this particular Interleaver.get https://github.com/jwodder/interleave/blob/master/src/interleave/__init__.py#L348 is the primary suspect due to internal `while True` loop.

@jwodder could you please see if code there potentially be made more efficient?