alternator: return tags for a table via const reference
alternator: return tags for a table via const reference

The signature of the helper function is changed, so that it's possible
to acquire a const reference of the tags, instead of being forced
to get a copy of the whole map (potentially large).

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