txn_proto2: don't cleanup chain on the critical path
txn_proto2: don't cleanup chain on the critical path

let the background tree walkers do this work

stephentu/silo,nathanielherman/silo,stephentu/silo,nathanielherman/silo,nathanielherman/silo,nathanielherman/silo,stephentu/silo,stephentu/silo