storage: de-flake TestLeaderLeaseConcurrent
storage: de-flake TestLeaderLeaseConcurrent

Reproposals were causing spurious test failures; the test now logs
a warning in this case (and tests less).
A missing return statement caused (much rarer) deadlock due to prompting
a possibly dead-blocking channel send.

Ran 110k+ iterations of this without issues; the failures would usually
manifest locally in <10k iterations.

Fixes #6182.

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