codeintel: Refactor data migrator

Created by: efritz

This refactors the generic code intelligence oob migrator so that:

• The migrations themselves can be unaware of dump ids
• Can actually tell what stuff is doing (major functionality broken into functions)
• We select records whose bounds need to be updated regardless if there are migrable records (this fixes a 97% complete-now-stuck bug in cloud)
• Fixes contention issue where multiple migrators will select rows from the same dump id (now we lock schema_version records, and not the records in the data table)

Please review migrator.go's new code only (not the diff). Other file's diffs are reasonable :)