Floor block count at 0

Fixes #653, #655

electrumx/server/block_processor.py

@@ -99,11 +99,11 @@ class Prefetcher(object):
         async with self.semaphore:
             while self.cache_size < self.min_cache_size:
                 # Try and catch up all blocks but limit to room in cache.
-                # Constrain fetch count to between 0 and 500 regardless;
-                # testnet can be lumpy.
+                # Constrain fetch count to between 0 and 100 regardless;
+                # some chains can be lumpy.
                 cache_room = max(self.min_cache_size // self.ave_size, 1)
                 count = min(daemon_height - self.fetched_height, cache_room)
-                count = min(500, count)
+                count = min(100, max(count, 0))
                 if not count:
                     self.caught_up = True
                     return False