Need batch read API that takes caller-supplied buffer to fill

The current batch read API, `GetN`, creates and returns a new populated buffer on every call. This does not allow the buffer to be reused across multiple calls to `GetN`, and may result in GC that could be avoided by buffer reuse.

To address this, there should be a new API that takes a buffer, provided by the caller, and drains the queue into that buffer. A call to this API returns when the provided buffer is full or when there are no more items in the queue to drain.