kv: fixing LogEvent hack in dist_sender
kv: fixing LogEvent hack in dist_sender

Replacing a direct use of Span.LogEvent with Eventf, which also doesn't format
the string if we aren't actually logging into a span.

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