Provide a service to handle transient storage

We have several cases where need to store some data temporarily :

• Asynchronous Renditions is one case
• Conversions is one case
• Asynchonous S3 BinaryManager

We may also want to have this kind of storage to handles some flags on documents :

• this document is being re-indexed
• this document is being moved
• ...

This transient storage is different from a simple case because we don't want do have the "cache entries" removed too early.
Typically :

• I should not remove the S3 cache entry before the actual S3 upload is completed
• I should not delete the rendition before someone reads it (or at least I should wait a few hours)

The idea is to base the implementation on a 2 levels cache system :

• L1 cache is long lived
• L2 cache is short lived

Entries are initially stored on L1 and moved in L2 when they have been used .