Change how invite accounting is done

[corrideat]

Problem

Currently, invites are used up immediately when OP_KEY_REQUEST is sent. This can be problematic because it can result in invites being used up when they shouldn't, such as if someone is spamming the group. It's also a bit inflexible, because not all key requests are necessarily valid.

Solution

Instead, we can use invites on OP_KEY_REQUEST_SEEN. The payload for OP_KEY_REQUEST_SEEN now is an encrypted blob, so we need to modify it to always include information that can be used to reduce the invite count. Two possibilities are:

1. Making keyRequestHash always cleartext (now, it's potentially encrypted)
2. In order to reveal less information, it could just have a cleartext field like keyId, so looking something like { keyId: string; payload: ProtoSPOpKeyRequestSeen | EncryptedData<ProtoSPOpKeyRequestSeen> }