[erlang-patches] Add mnesia activity event subscription

[Bernard Duggan]

Hi Björn et al,
    I've modified the activity event message to be more consistent with
the other mnesia event message types and also added documentation
describing the event type to the mnesia user's guide.

Changes are on the same branch:
git fetch git://github.com/bernardd/otp.git activity_subscribe

Cheers,

Bernard