sstables: Don't register logger with the same name twice
sstables: Don't register logger with the same name twice

There can be one logger with given name. This was causing
--logger-log-level sstable=trace to not work for the majority of log
points.

Message-Id: <d3b610604174eb83c22846ff9263d43cf0ede3fb@scylladb.com>

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