Introduce a layered store for archive nodes

To benefit from the recent optimizations introduced by irmin.3.4 (GC), archive nodes should implemented with a:

• upper layer with all the live blocks -- which consists in a few cycles only
• lower layer with all the blocks that are below the last allowed fork level

This should be implemented in the next release of irmin: 3.7.

This is expected to improve the performances of the archives nodes while querying data from the live blocks. The access to non live blocks won't be optimized unless the lower store is split into various volumes (a feature that should be implemented in irmin.3.7).