Add low level support for Async Renditions

Renditions are more and more used inside Nuxeo :

• because when using CMIS this is very useful to access some features of Nuxeo
• because we are improving the concept of renditions inside Nuxeo

However, renditions can be costly to generate so we can not always generated them "on the fly".
On the other hand, we may not always want to pre-compute it :

• it may be inefficient
  • only a few part of the renditions will be accessed
  • renditions are very short lived
• it may create security issues
  • renditions depends on the user security context

The idea is to extend the framework to provide a way to handle with such cases :

• provide a way to declare a rendition as "asynchronous"
• provide an infrastructure for handling computation
• provide a transient store to handle the result