Counterparty identifier can't be modified

[maxalbert]

Flowmachine metrics allow choosing the subscriber identifier (e.g. msisdn, imei). However, some metrics also involve the counterpart and this is always msisdn. This is partly a limitation, but partly also leads to erroneous logic because self-calls can't be excluded if subscriber_identifier = 'imei'. This should be fixed at the schema level by a separate subscriber table.