v1.9.1-beta01: Fix synchronizer stop, scan loop, and SQLite isolation

- Fix SdkSynchronizer.stop() race condition: use synchronous scope.cancel()
  instead of async self-cancellation that never completed
- Fix scan loop: scan once after all downloads complete instead of per-batch
- Use fresh SQLiteDatabase connection for getLastScannedHeight() to avoid
  reading stale Room-cached values
- Bump version to 1.9.1-beta01
- Update Cargo dependencies

gradle.properties

@@ -21,7 +21,7 @@ RELEASE_SIGNING_ENABLED=false
 # Required by the maven publishing plugin
 SONATYPE_HOST=DEFAULT
 
-LIBRARY_VERSION=1.9.0-beta01
+LIBRARY_VERSION=1.9.1-beta01
 
 # Kotlin compiler warnings can be considered errors, failing the build.
 # Currently set to false, because this project has a lot of warnings to fix first.