SOLR datasource

[richard-jones]

• PR: [when the PR is created, put a link to it here. DO NOT use the "linked PR" feature]

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Implement a datasource that allows us to talk to SOLR. It will need to provide a query adapter and convert the result set back to the internal edges format (which is essentially elasticsearch).

Acceptance Criteria

• A typical edges interface can be run over a SOLR instance