x/staking: add call to iterator Close() method (#6794)

Original pull request: #6791

CHANGELOG.md

@@ -44,6 +44,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (client) [\#5585](https://github.com/cosmos/cosmos-sdk/pull/5585) `CLIContext` additions:
   * Introduce `QueryABCI` that returns the full `abci.ResponseQuery` with inclusion Merkle proofs.
   * Added `prove` flag for Merkle proof verification.
+* (x/staking) [\#6791)](https://github.com/cosmos/cosmos-sdk/pull/6791) Close {UBDQueue,RedelegationQueu}Iterator once used.
 
 ### API Breaking Changes
 

x/staking/keeper/delegation.go

@@ -258,6 +258,8 @@ func (k Keeper) DequeueAllMatureUBDQueue(ctx sdk.Context,
 	store := ctx.KVStore(k.storeKey)
 	// gets an iterator for all timeslices from time 0 until the current Blockheader time
 	unbondingTimesliceIterator := k.UBDQueueIterator(ctx, ctx.BlockHeader().Time)
+	defer unbondingTimesliceIterator.Close()
+
 	for ; unbondingTimesliceIterator.Valid(); unbondingTimesliceIterator.Next() {
 		timeslice := []types.DVPair{}
 		value := unbondingTimesliceIterator.Value()
@@ -445,6 +447,8 @@ func (k Keeper) DequeueAllMatureRedelegationQueue(ctx sdk.Context, currTime time
 	store := ctx.KVStore(k.storeKey)
 	// gets an iterator for all timeslices from time 0 until the current Blockheader time
 	redelegationTimesliceIterator := k.RedelegationQueueIterator(ctx, ctx.BlockHeader().Time)
+	defer redelegationTimesliceIterator.Close()
+
 	for ; redelegationTimesliceIterator.Valid(); redelegationTimesliceIterator.Next() {
 		timeslice := []types.DVVTriplet{}
 		value := redelegationTimesliceIterator.Value()