fix: send rewards in Unstake at once

[hallazzang]

Currently we call WithdrawRewards for each staking coin denom, which sends rewards for that denom to the farmer:

farming/x/farming/keeper/staking.go

Lines 280 to 301 in debb2e6

	for _, coin := range amount {
	staking, found := k.GetStaking(ctx, coin.Denom, farmerAcc)
	if !found {
	staking.Amount = sdk.ZeroInt()
	}
	queuedStaking, found := k.GetQueuedStaking(ctx, coin.Denom, farmerAcc)
	if !found {
	queuedStaking.Amount = sdk.ZeroInt()
	}
	availableAmt := staking.Amount.Add(queuedStaking.Amount)
	if availableAmt.LT(coin.Amount) {
	return sdkerrors.Wrapf(
	sdkerrors.ErrInsufficientFunds, "%s%s is smaller than %s%s", availableAmt, coin.Denom, coin.Amount, coin.Denom)
	}
	queuedStaking.Amount = queuedStaking.Amount.Sub(coin.Amount)
	if queuedStaking.Amount.IsNegative() {
	if _, err := k.WithdrawRewards(ctx, farmerAcc, coin.Denom); err != nil {
	return err
	}

We can optimize this to send total rewards only once at the end of the function.
To do this, we should change WithdrawRewards to not send coins, instead return just sdk.Coins.

But we need to discuss more about this optimization. It just reduces gas cost of MsgUnstake.