Remove hard MongoDB dependencies from pub-auth-service and pub-issue-service

[wkerckho]

The modules pub-auth-service and pub-issue-service directly interface with MongoDB via the Vert.x Mongo client, which is problematic when users want to e.g. swap out the MetaStore implementation.

For pub-auth-service, a Mongo connection is used for the KeyStore, maybe we should integrate this functionality in the IdentityStore interface?

For pub-issue-service, this is more difficult, as I don't see issues as a core part of the Obelisk API. Maybe this module should be seen as an extension and be removed from the main repo?

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