fix: (x/feegrant) expiration when overwriting an existing grant

[atheeshp]

Description

This PR couple of bugs with expiration not being updated when we overwrite an existing grant with different expiry.

Bug 1
grantee, granter passed wrongly to the getGrant method here:

cosmos-sdk/x/feegrant/keeper/keeper.go

Line 55 in a187a1c

existingGrant, err := k.getGrant(ctx, grantee, granter)

grantee is passed at granter's place and vice versa

cosmos-sdk/x/feegrant/keeper/keeper.go

Lines 181 to 196 in a187a1c

	// getGrant returns entire grant between both accounts
	func (k Keeper) getGrant(ctx sdk.Context, granter sdk.AccAddress, grantee sdk.AccAddress) (*feegrant.Grant, error) {
	store := ctx.KVStore(k.storeKey)
	key := feegrant.FeeAllowanceKey(granter, grantee)
	bz := store.Get(key)
	if len(bz) == 0 {
	return nil, sdkerrors.Wrap(sdkerrors.ErrUnauthorized, "fee-grant not found")
	}
	var feegrant feegrant.Grant
	if err := k.cdc.Unmarshal(bz, &feegrant); err != nil {
	return nil, err
	}
	return &feegrant, nil
	}

Bug 2
The following if condition never gets satisfied, getGrant returns err, grant with two possible out comes which are nil, grant or err, nil

cosmos-sdk/x/feegrant/keeper/keeper.go

Line 56 in a187a1c

if err != nil && existingGrant != nil && existingGrant.GetAllowance() != nil {

and also there is not only one possible error from getGrant with fee-grant not found and also one another possible error is unmarshal error, when there is an unmarshal error we should return the error.

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[likhita-809]

lgtm

[alexanderbez]

@atheeshp can you look into the failing feegrant test?

    --- FAIL: TestIntegrationTestSuite/TestNewCmdRevokeFeegrant (11.80s)