Same store key for nextSequenceSend in v1 and v2 channels

[gjermundgaraba]

nextSequenceSend should be the same key for v1 and v2 channels.

This means we need to change v1 to use the same key path as v2 and have a store migration for that.

Make sure we have tests that cover this before implementing this change. We want to make sure we don't regress on this.

We should also use the same codepaths for this, so we don't have getting/setting nextSequenceSend and the keys in multiple places (i.e. reuse the upgraded paths from v1)