Redesign instanciation of Irmin stores

[g2p]

Because of the Irmin.Private.Conf bottleneck, which doesn't accept custom types, it is not possible to pass around an already initialized disk. Hack around that by making the config construct stateful, associating a disk to a path, and reusing that when a config with the same path is passed later.

This will at least remove the need for passing custom EXTBLOCK signatures.