WebSession: fix for merge_usera_into_userb()

• Fixes a corner case in merge_usera_into_userb() usage where a user could have been "merged with himself" when his known email was stored in the database with upper and lower case characters.

• Introduces un upgrade recipe to make all the email addresses stored in the DB lower case.

• Disables merging for rnkPAGEVIEWS, rnkDOWNLOADS, and session tables since these are usually huge and they are not really critical from the point of view of record merging.

• Disables locking of affected tables upon merging, at the very low risk of inconsistencies, but without the risk of locking up the service.