Rename protocol to ProtocolName; the former is already used

[ltning]

This change allows TCP and UDP to be correctly processed by the GELF input.

Previously, the 'protocol' field was used by lower-level protocols, causing errors in graylog:

[136]: index [graylog2_1590], type [_doc], id [91994481-95ee-11ee-84f1-02ce55e8ca0b], message [ElasticsearchException[Elasticsearch exception [type=mapper_parsing_exception, reason=failed to parse field [protocol] of type [long] in document with id '91994481-95ee-11ee-84f1-02ce55e8ca0b'. Preview of field's value: 'TCP']]; nested: ElasticsearchException[Elasticsearch exception [type=illegal_argument_exception, reason=For input string: "TCP"]];]