Add mnesia activity event subscription

[Bernard Duggan]

Hi all,
    Following a discussion over on erlang-questions, I've implemented an
additional mnesia subscription type called 'activity'. 

This feature allows a process that is watching changes through the
mnesia:subscribe({table, _, _}) system to know when a particular
activity has completed and therefore to group changes received through
the subscription system into groups corresponding to a single
transaction.  While this grouping by ActivityID was previously possible,
it was not possible to know when the /last/ change for a given
transaction had been received and therefore be able to forward on a
complete transaction change set to some other process.

What I'm looking for here is feedback rather than immediate inclusion -
This patch is currently not OTP-worthy, if only because it needs
documentation - I didn't want to spend too much time on that aspect,
though, if the judgement is going to be that this is a totally
inappropriate feature or I've gone about it in entirely the wrong way :)

On a related note, is there a test suite for mnesia?  I couldn't find
one that I might be able to add test cases to.

git fetch git://github.com/bernardd/otp.git activity_subscribe

Cheers,

Bernard