maps: do not ask sqlite for row count "SELECT COUNT(1) FROM x" is not O(1) for sqlite and can be slow on large tables. This patch changes the count to be backed by a file instead. The change exposes a risk that the number may become inaccurate, if __setitem__ is called with a same key multiple times. But we don't do that during pull, and only use __len__ to calculate how many revisions pulled, or test if the map is empty. So it would be fine.