add state snapshot uploads alongside log uploads

[snowp]

Implements state uploads by introducing a new StateUploadWorker that is notified whenever a log batch is uploaded and attempts to schedule the upload of any state snapshots that are necessary in order to provide the state updates required for hydrating the uploaded logs.

This is a generic mechanism that ends up working for both trigger buffer uploads as well as continuous uploads:

• for trigger buffers the snapshots are kept on disk subject to the retention policy given by the data contained in the buffers, while uploads are scheduled whenever we observe logs being uploaded
• for continuous buffers we rely on the upload signals to notify the worker about logs that are being uploaded, which may require periodic snapshot rotation in order to prepare the snapshots for upload