Do Spar things transactionally

Right, I think this might still be worth it in this one, and should look like something in the lines of:

insertScimAndSamlUser :: (HasCallStack, MonadClient m) => UserId -> SAML.UserRef -> ScimC.User.StoredUser SparTag -> m ()
insertScimAndSamlUser uid (SAML.UserRef tenant subject) scimUsr = retry x5 . batch $ do
  setType BatchLogged
  setConsistency Quorum
  addPrepQuery insScim (uid, WrappedScimStoredUser scimUsr)
  addPrepQuery insSaml (tenant, subject, uid)
  where
    insScim :: PrepQuery W (UserId, WrappedScimStoredUser SparTag) ()
    insScim = "INSERT INTO scim_user (id, json) VALUES (?, ?)"

    insSaml :: PrepQuery W (SAML.Issuer, SAML.NameID, UserId) ()
    insSaml = "INSERT INTO user (issuer, sso_id, uid) VALUES (?, ?, ?)"

The main annoyance I see with this is that you will need a Show instance for WrappedScimStoredUser SparTag -> not sure how trivial it is to come up with a meaningful one.

Originally posted by @tiago-loureiro in https://github.com/wireapp/wire-server/pull/905

Review the proposed insertScimAndSamlUser definition and the referenced pull request #905 first. Trace the existing Spar SCIM and SAML insertion path, including the batch and retry behavior shown in the issue. Done means the two inserts are handled transactionally as described and the WrappedScimStoredUser SparTag serialization concern is resolved.