_field_names should not get docvalues

[rmuir]

When i try to reproduce @peterskim12 benchmark here (https://github.com/peterskim12/elk-index-size-tests), I see larger space usage for master than expected.

I first looked at fieldinfos and see _field_names with SORTED_SET, this should be pretty wasteful and probably is the smoking gun:

field	indexed	docvalues
_uid	DOCS	NONE
_source	NONE	NONE
_type	DOCS	NONE
_version	NONE	NUMERIC
@Version	DOCS	SORTED_SET
@timestamp	DOCS	SORTED_NUMERIC
host	DOCS_AND_FREQS_AND_POSITIONS	NONE
host.raw	DOCS	SORTED_SET
clientip	DOCS_AND_FREQS_AND_POSITIONS	NONE
clientip.raw	DOCS	SORTED_SET
ident	DOCS_AND_FREQS_AND_POSITIONS	NONE
ident.raw	DOCS	SORTED_SET
auth	DOCS_AND_FREQS_AND_POSITIONS	NONE
auth.raw	DOCS	SORTED_SET
timestamp	DOCS_AND_FREQS_AND_POSITIONS	NONE
timestamp.raw	DOCS	SORTED_SET
verb	DOCS_AND_FREQS_AND_POSITIONS	NONE
verb.raw	DOCS	SORTED_SET
request	DOCS_AND_FREQS_AND_POSITIONS	NONE
request.raw	DOCS	SORTED_SET
httpversion	DOCS_AND_FREQS_AND_POSITIONS	NONE
httpversion.raw	DOCS	SORTED_SET
response	DOCS	SORTED_NUMERIC
bytes	DOCS	SORTED_NUMERIC
referrer	DOCS_AND_FREQS_AND_POSITIONS	NONE
referrer.raw	DOCS	SORTED_SET
agent	DOCS_AND_FREQS_AND_POSITIONS	NONE
agent.raw	DOCS	SORTED_SET
_all	DOCS_AND_FREQS_AND_POSITIONS	NONE
_field_names	DOCS	SORTED_SET

[rmuir]

I peeked at the docvalues file and saw ~ 8MB ordinals alone for _field_names, so it accounts for the space increase that surprised me (75MB -> 83MB total index size).