Serialize only the record key when logging an AerospikeError

Fixes #1076

AerospikeError keeps a reference to the failed command, and the command holds the client and the request arguments. JSON.stringify, pino and winston serialized all of it. Logging a failed batchWrite copied the whole payload into the log line, along with the client config and its password (#415).

This adds Command#toJSON, which returns only the record key. error.command and error.client still return the same objects. Serialized errors now show command as { key }.