esplora: fix incorrect gap limit check in blocking client

esplora: fix incorrect gap limit check in blocking client

The gap limit was checked such as if the last_index was not None but the
last_active_index was, we'd consider having reached it. But the
last_index is never None for this check. This effectively made it so the
gap limit was always 1: if the first address isn't used
last_active_index will be None and we'd return immediately.

Fix this by avoiding error-prone Option comparisons and correctly
checking we've reached the gap limit before breaking out of the loop.