Generalize "insert into DB though sorted files" pattern. (#592)

cmd/state/generate/regenerate_index.go

cmd/state/verify/check_indexes.go

common/changeset/changeset.go

common/etl/etl.go

common/etl/etl_test.go

common/etl/heap.go

core/generate_index.go

core/generate_index_test.go

eth/downloader/stagedsync_stage_hashcheck.go

eth/downloader/stagedsync_stage_hashcheck_test.go

eth/downloader/stagedsync_stage_indexes.go

go.mod

go.sum