irmin-pack: ensure snapshot-isolation of read-only instances

[samoht]

This is the counterpart of mirage/index#198

We'd like irmin-pack read-only instances to always stay valid, whatever happens. E.g. a concurrent clear + adds should not lead to valid reads in the read-only instance. To see any new values, the read-only instance should use sync.

This is a bit different from the other backends, where read-only instance can see when values are removed/added without the need for an explicit sync. But this is needed to ensure that 1/ irmin-pack read-only instances do not keep reading the file headers to check if the file have been clearer; and 3/ irmin-pack read-only instances always read valid data.

[samoht]

@icristescu is this fixed?

[icristescu]

Indeed, this is covered by the tests here, here and here.