Map claimed key packages to clients

[pcapriotti]

This PR implements a mapping of key package ref to qualified clients. The corresponding cassandra table also contains fields to hold the qualified conversation ID of where the key package is being used. These fields are not set for the moment, but they will play a role when validating incoming handshake messages.

This also adds an internal brig endpoint to dereference a key package. This is only used in tests for the moment, but it will later be used by galley to resolve a key package ref to a user and client ID.

Part of https://wearezeta.atlassian.net/browse/FS-436.

Note: this contains a commit from #2170. Rebase it away before merging.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.