alternator: use trace level for debug messages
alternator: use trace level for debug messages

In the early development stage, warn level was used for all
debug messages, while it's more appropriate to use 'trace' or 'debug'.
Message-Id: <a9a7be5173f424be4dc931cbc73c9e1a438c321e@scylladb.com>

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