fix(server): correct batch materialization logic in BatchAccumulator

[hubcio]

This commit refactors the BatchAccumulator by removing the
capacity field and simplifying the logic for materializing
batches. The materialize_batch_and_maybe_update_state method
is renamed to materialize_batch_and_update_state, and the
logic for handling message offsets and timestamps is streamlined.

The changes improve the clarity and efficiency of the batch processing
logic, addressing issues with the previous handling of has_remainder.
Additionally, the logic in writing_messages.rs is updated to reflect
these changes, ensuring that unsaved messages are correctly managed
based on batch size.

Now, instead of having hard-limit of messages_required_to_save we
will trigger it once when capacity if exceeded and save all messages
on disk, instead of handling remainder.