Coverage cleanup #118

This contains several changes:

• bring back issued-year field, used to break down by issued year for coverage
• use correct field in type-name facet transformer
• make sure that journal metadata does not contain counts-type field, and coverage-type field is not broken down by content type
• make sure coverage index contains only the latest coverage object for every member and journal
• when open references fraction is calculated for a member/journal, it uses list of open prefixes calculated once for thee entire coverage task (for performance reasons)
• when statistics for content types are calculated for a member/journal, only relevant content types are processed (for performance reasons)
• use offsets for processing the entire members and journals sets