Support online and batched queries simultaneously

[tholop]

The current code exposes two separate APIs, one for online queries and one for batched queries. In theory we could expose a single API, that interleaves online and batched queries.

[roxanageambasu]

Maybe we should remove the batching for now to simplify the lib?

[tholop]

Sounds good to me. Most of the batching code is in batch_pds and is only loosely integrated with the rest of the library. Removing that will also simplify consumers (e.g., experiments) that depend on pdslib. @giorgi-o what do you think?

For future reference, the most recent batch code remains available in the June 2025 release (https://github.com/columbia/pdslib/releases/tag/v0.3). It hasn't changed since then apart from minor cleanups.