storage: fix race between context cancel & snapshots
storage: fix race between context cancel & snapshots

In (*RaftTransport).ProcessQueue(), the context for the gRPC calls was
being cancelled as part of a defer block. Unfortunately, errors (which
happen too frequently) cause the method to return and cancel outstanding
Raft requests, including snapshots, prematurely.

This change introduces a WaitGroup that blocks the cancellation until
the outstanding Raft requests have finished. This speeds up
up-replication by ~40-50%.

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