ConnectionType, getOrCreateCampaignEnsureUrlKey()

Modify domain and persistence classes to use ConnectionType to set
whether DB_MASTER or DB_SLAVE is used, instead of boolean
parameters, and add
ICampaignRepository::getOrCreateCampaignEnsureUrlKey()
to guarantee a campaign for a given URL key.

Change-Id: I3d315b24cd07a0ae00d265401fe4f49973042fca