server: fix snowball recorder in node.Batch
server: fix snowball recorder in node.Batch

The snowball recorder in Batch assumes all spans (including child spans)
complete before the request completes. This may not necessarily be the case
(e.g. #10279). The direct use of `br` is also error-prone.

We fix this by collecting the spans in a synchronized structure and discarding
any late spans.

glycerine/cockroach,paperstreet/cockroach,glycerine/cockroach,maxlang/cockroach,paperstreet/cockroach,paperstreet/cockroach,maxlang/cockroach,glycerine/cockroach,petermattis/cockroach,veteranlu/cockroach,paperstreet/cockroach,maxlang/cockroach,veteranlu/cockroach,petermattis/cockroach,glycerine/cockroach,petermattis/cockroach,vaibhavarora/cockroach,maxlang/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,glycerine/cockroach,vaibhavarora/cockroach,petermattis/cockroach,maxlang/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,glycerine/cockroach,maxlang/cockroach,glycerine/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,maxlang/cockroach,petermattis/cockroach,paperstreet/cockroach,petermattis/cockroach,glycerine/cockroach,petermattis/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,petermattis/cockroach,veteranlu/cockroach,veteranlu/cockroach,maxlang/cockroach,veteranlu/cockroach,vaibhavarora/cockroach