storage/engine: enable max_subcompactions
storage/engine: enable max_subcompactions

Set max_subcompations to `num_cpu/2`.

arjun4084346/cockroach,petermattis/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,petermattis/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,veteranlu/cockroach,veteranlu/cockroach,glycerine/cockroach,arjun4084346/cockroach,glycerine/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,glycerine/cockroach,paperstreet/cockroach,veteranlu/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,sunya123/cockroach,vaibhavarora/cockroach,sunya123/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,sunya123/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,paperstreet/cockroach,veteranlu/cockroach,veteranlu/cockroach,glycerine/cockroach,maxlang/cockroach,maxlang/cockroach,petermattis/cockroach,vaibhavarora/cockroach,sunya123/cockroach,maxlang/cockroach,paperstreet/cockroach,arjun4084346/cockroach,glycerine/cockroach,maxlang/cockroach,cockroachdb/cockroach,sunya123/cockroach,cockroachdb/cockroach,petermattis/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,glycerine/cockroach,vivekmenezes/cockroach,petermattis/cockroach,glycerine/cockroach,maxlang/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,cockroachdb/cockroach,petermattis/cockroach,sunya123/cockroach,arjun4084346/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,sunya123/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,sunya123/cockroach,maxlang/cockroach,glycerine/cockroach,petermattis/cockroach,maxlang/cockroach,petermattis/cockroach,veteranlu/cockroach,maxlang/cockroach,vaibhavarora/cockroach