Reset cached statements after using them

[simolus3]

This fixes two instances of a pending statement being busy after a powersycnc_control invocation, which would make some database operations (like a vacuum) impossible.

1. In StorageAdapter.now, reset the statement after running it.
2. Refactor StorageAdapter.local_progress to not use streaming iterators (they're tricky here because closing them can't return an error code). Instead, we make the prepared statement public and add a method to step and interpret typed results.

To test this across platforms, I've adopted the sqlite3_native_assets package in tests. It relies on native assets to compile and link SQLite instead of using the one from the operating system. This allows us to enable the statement vtab, used in tests to assert no statements remain busy in the end. Actually I'll probably have to revert that because it means we can't test with a stable SDK. Not sure how we'd test regressions then.