database: snapshot_on_all: flush before snapshot if needed
database: snapshot_on_all: flush before snapshot if needed

flush_on_all shards before taking the snapshot if !skip_flush
so we can get rid of flushing in table::snapshot.

Refs #10500

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla