irmin-pack: integrity_check minimal indexing stores

[icristescu]

When importing a snapshot with the current code, there is an implicit integrity check: the objects are added to store bottom up, their hash are computed and the resulting hash is compared with the expected hash written in the snapshot file.

If we switch to the approach in #2125 , the import will consists in opening a store and there is no implicit integrity check anymore. We should add an option to do the integrity check when opening a store or add a separate integrity check function in the API, so that users can call it explicitly.

Note that an integrity check function currently exists, but it uses the index to traverse the pack file. This is not enough for stores with minimal indexing strategy (which is the case of the stores using the gc-based snapshots).