Updating group attributes causes database error

Updating a groups's attributes requires a remove then add of common attributes. Two inserts can happen simultaneously due to the read committed isolation level on the database. This error only manifests itself under high concurrent mutation of the attributes on the exact same group (ie. extremely rare).

We currently do not expose group attributes via the Crowd Console UI, but there's potential here for errors to occur if users have extended Crowd to access these attributes.