storage: queue requests to push txn / resolve intents on single keys

Previously, high contention on a single key would cause every thread to
push the same conflicting transaction then resolve the same intent in
parallel. This is inefficient as only one pusher needs to succeed, and
only one resolver needs to resolve the intent, and then only one writer
should proceed while the other readers/writers should in turn wait on
the previous writer by pushing its transaction. This effectively
serializes the conflicting reader/writers.

One complication is that all pushers which may have a valid, writing
transaction (i.e., `Transaction.Key != nil`), must push either the
conflicting transaction or another transaction already pushing that
transaction. This allows dependency cycles to be discovered.

pkg/storage/batcheval/cmd_push_txn.go

@@ -205,8 +205,8 @@ func PushTxn(
 		if !pusherWins {
 			s = "failed to push"
 		}
-		log.Infof(ctx, "%s "+s+" %s: %s (pushee last active: %s)",
-			args.PusherTxn.Short(), args.PusheeTxn.Short(),
+		log.Infof(ctx, "%s "+s+" (push type=%s) %s: %s (pushee last active: %s)",
+			args.PusherTxn.Short(), args.PushType, args.PusheeTxn.Short(),
 			reason, reply.PusheeTxn.LastActive())
 	}