Allow snapshotting multiple repositories

Transactions currently snapshot the target repository and the alternate repository if it has one. This allows transactions to operate on repositories that already have an alternate but doesn't yet make it possible add new alternate links. To do so, we'd need to be able to start a transaction covering both the target repository, and the alternate repository it is about to be linked to.

This commit makes it possible to start a transaction covering multiple repositories. The transaction has a single target repository where changes are recorded. In addition, multiple other repositories can be included in the snapshot. Changes into these repositoris are not recorded or committed as we don't have a need to do so yet.

With the pool and the new member in the snapshot, the RPC handler managing alternate links can create the link and verify it in its own snapshot, and stage the change for committing. This will be later implemented later.