snapshots: don't hash pending snapshots by snapshot name
snapshots: don't hash pending snapshots by snapshot name

If we are hashing more than one CF, the snapshot themselves will all have the same name.
This will cause the files from one of them to spill towards the other when writing the manifest.

The proper hash is the jsondir: that one is unique per manifest file.

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@scylladb.com>

kjniemi/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,raphaelsc/scylla,kjniemi/scylla,avikivity/scylla,raphaelsc/scylla,avikivity/scylla,duarten/scylla,duarten/scylla