re-enable txn auto gc
re-enable txn auto gc

fixes #2717 (ok since #2799 has landed).

mbertschler/cockroach,nieyy/cockroach,petermattis/cockroach,vivekmenezes/cockroach,maxlang/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,petermattis/cockroach,sunya123/cockroach,vivekmenezes/cockroach,il9ue/cockroach,maxlang/cockroach,sunya123/cockroach,cuongdo/cockroach,glycerine/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,veteranlu/cockroach,mbertschler/cockroach,cuongdo/cockroach,mbertschler/cockroach,cockroachdb/cockroach,nieyy/cockroach,vaibhavarora/cockroach,il9ue/cockroach,bg451/cockroach,il9ue/cockroach,glycerine/cockroach,vivekmenezes/cockroach,marcuswestin/cockroach,arjun4084346/cockroach,paperstreet/cockroach,nieyy/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,glycerine/cockroach,petermattis/cockroach,cockroachdb/cockroach,sunya123/cockroach,nieyy/cockroach,arjun4084346/cockroach,vivekmenezes/cockroach,petermattis/cockroach,paperstreet/cockroach,cuongdo/cockroach,cuongdo/cockroach,petermattis/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,glycerine/cockroach,il9ue/cockroach,nieyy/cockroach,marcuswestin/cockroach,sunya123/cockroach,cuongdo/cockroach,sunya123/cockroach,bg451/cockroach,cuongdo/cockroach,glycerine/cockroach,il9ue/cockroach,maxlang/cockroach,glycerine/cockroach,veteranlu/cockroach,bg451/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,marcuswestin/cockroach,paperstreet/cockroach,marcuswestin/cockroach,maxlang/cockroach,arjun4084346/cockroach,cuongdo/cockroach,mbertschler/cockroach,veteranlu/cockroach,nieyy/cockroach,veteranlu/cockroach,petermattis/cockroach,glycerine/cockroach,vaibhavarora/cockroach,il9ue/cockroach,arjun4084346/cockroach,maxlang/cockroach,veteranlu/cockroach,marcuswestin/cockroach,maxlang/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,il9ue/cockroach,cockroachdb/cockroach,paperstreet/cockroach,sunya123/cockroach,marcuswestin/cockroach,veteranlu/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,bg451/cockroach,maxlang/cockroach,marcuswestin/cockroach,sunya123/cockroach,arjun4084346/cockroach,marcuswestin/cockroach,cuongdo/cockroach,glycerine/cockroach,il9ue/cockroach,bg451/cockroach,petermattis/cockroach,arjun4084346/cockroach,petermattis/cockroach,mbertschler/cockroach,bg451/cockroach,maxlang/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,mbertschler/cockroach,cockroachdb/cockroach,sunya123/cockroach,cockroachdb/cockroach,nieyy/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,nieyy/cockroach,bg451/cockroach,mbertschler/cockroach,paperstreet/cockroach