Fix off-by-one error in shards, remove asserts

Previously was including one extra IP per shard due because the last IP
was erroroneously included in the shard, when it should be an open
interval.

Removed debug asserts that were added when testing the specific case of
3 shards, 1 thread
Fix calculation of begin for sharding with threads

begin_idx = shard_num + thread_num*num_shards
                                   ^not num_threads