Irmin-pack: add stats for reads in suffix/prefix

[icristescu]

In the dispatcher we have two types of reads - in the prefix or in the suffix. We should add stats to track those.

Also in the pack_store, we can read sometimes try to read gced objects, for instance when trying to find them. This can also be tracked by a counter.

[samoht]

That might be useful to verify the assumption that Tezos should read very rarely in the prefix - but not a blocker for the initial release.