Use stage name as db key (#858)

cmd/integration/commands/reset_state.go

cmd/integration/commands/root.go

cmd/pics/state.go

cmd/state/stateless/state_snapshot.go

common/dbutils/bucket.go

common/etl/etl.go

core/pruner.go

core/state/history.go

core/state/history_test.go

eth/backend.go

eth/stagedsync/stages/stages.go

ethdb/interface.go

ethdb/kv_abstract.go

ethdb/kv_badger.go

ethdb/kv_bolt.go

ethdb/kv_lmdb.go

ethdb/kv_remote.go

ethdb/kv_remote2.go

ethdb/object_db.go

migrations/migrations.go