Generation of unique IDs

[Brent Pinkney]

> Until then, you should consider having one MagmaSession for each seaside  
> session.

That is precicely what we have implemented - it works beautifully.

Allocating a new MagmaSesison is quite slow as it must perofrm a lot of  
consistency checks on the image to confirm it is compatible with the  
objects in the repository. This can make the Seaside app look sluggish as  
allocating new Seaside session is a pretty frequent occurence.

A nice optmisation is to always have a MagamSession pre-allocated which  
can be used by a new Seaside session. Another MagmaSession can then be  
allocated asynchronously.

Brent