snapshots: separate manifest creation
snapshots: separate manifest creation

The way manifest creation is currently done is wrong: instead of a final
manifest containing all files from all shards, the current code writes a
manifest containing just the files from the shard that happens to be the
unlucky loser of the writing race.

In preparation to fix that, separate the manifest creation code from the rest.

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

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