exponential backoff

lib/server.js

@@ -761,7 +761,7 @@ WalletService.prototype._canCreateTx = function(copayerId, cb) {
     var lastTxTs = txs[0].createdOn;
     var now = Math.floor(Date.now() / 1000);
     var timeSinceLastRejection = now - lastTxTs;
-    var backoffTime = WalletService.backoffTime * lastRejections.length;
+    var backoffTime = Math.pow(WalletService.backoffTime,lastRejections.length);
 
     if (timeSinceLastRejection <= backoffTime)
       log.debug('Not allowing to create TX: timeSinceLastRejection/backoffTime', timeSinceLastRejection, backoffTime);