enable docvalues for source_id field

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draft PR to test the effect of enabling docvalues for the source_id field.
this is motivated by the discussion in pelias/api#1608

it is hoped that this field can be used as an additional sorting criteria in order to make the ordering of results with the same _score value more deterministic, and therefore make testing more stable and predictable.

my concerns with this change are:

• greatly increasing the index size (on disk)
• increasing the memory consumption (at runtime once docvalues are loaded)

the source_id values are almost entirely unique and non-sequential, so I'd expect to see poor compression.

although.. these concerns are hopefully unwarranted, we might want to check those before merging this.

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