sqlite: DbResponse: add start_index

To avoid OOM situations, we want to return a fixed amount of memory,
rather than a fixed number of rows when querying the DB

We move to indexing into the collection of rows, rather than a fixed
per-page basis

start_index is the index into the db result where the current response
was generated from.

This is useful to ensure that the cursor's state is correct

proto/sqlite.proto

@@ -28,4 +28,5 @@ message DBResponse {
 	DBResult result = 1;
 	int32 sequenceNumber = 2;
 	int32 rowCount = 3;
+	int64 startIndex = 4;
 }