Fix replays across same-shard domain boundaries

[jonhoo]

If two adjacent domains are sharded the same way, but have a replay path across them that uses a different key from the sharding key, this currently leads to sadness. The code has some asserts that try to figure out if this is happening and crash appropriately, but we should really support that use-case. It will likely require a cross-shard replay, but routing the responses to only the appropriate shard will be a challenge (since the replay wouldn't go through a sharder).