Track downloaded operations in ps_buckets

[simolus3]

This alters the ps_buckets table to add two new columns:

• count_at_last: The amount of operations expected in a bucket, as received from the sync service. Set by client SDKs when completing a checkpoint.
• count_since_last: The amount of operations downloaded since the last full checkpoint. This is set by the core extension when receiving an update batch.

Having this information available allows client SDKs to calculate the progress we've made (we can display the total target as new_checkpoint.$bucket.count - ps_buckets.$bucket.count_at_last and the current progress as ps_buckets.$bucket.count_since_last).

I have considered setting count_at_last in sync_local (by just adding count_since_last to it, and then resetting count_since_last). However, I didn't like the possibility of count_at_last ever going out of sync with the count value the sync service gives us for buckets. That shouldn't be possible because we only sync local data when we have received all operations, but applying the value straight from the source still sounds more reliable to me.

Since I sometimes loose sight of which table does what in the core extension, I've also started a document in docs/ that's supposed to explain the role of local data eventually. It only mentions parts of ps_buckets for now, maybe we can expand on that further in the future.