get_user_by_email potentially slow

[ale-rt]

With lots of user the function get_user_by_email can be potentially slow on sites with lots of user because of the call to pm.searchMembers('email', email).
Anyway such sites can potentially custom PAS plugins/addons/customizations that might be used to perform the task in a more performant way (e.g. ldap/SQL/membrane_catalog queries).

For this reason it would be great to have, in the long run, that task performed by a utility view, but in the meanwhile adapters can be used to customize that function behavior.