RootMultiStore should reject duplicate storekey name when mount

[mossid]

It is possible to mount multiple storekeys with same name on RootMultiStore. We should check if the new storekey has unique name before we mount it.

Before:

//...
        if _, ok := rs.storesParams[key]; ok {
		panic(fmt.Sprintf("rootMultiStore duplicate store key %v", key))
	}
	rs.storesParams[key] = storeParams{
		key: key,
		typ: typ,
		db:  db,
	}
	rs.keysByName[key.Name()] = key
//...

After:

        if _, ok := rs.storesParams[key]; ok {
		panic(fmt.Sprintf("rootMultiStore duplicate store key %v", key))
	}
        for k := range rs.storeParams {
                if k.Name() == key.Name() {
                        panic(fmt.Sprintf("rootMultiStore duplicate store key name %v", key))
                }
        }
	rs.storesParams[key] = storeParams{
		key: key,
		typ: typ,
		db:  db,
	}
	rs.keysByName[key.Name()] = key

Also see: #2037

[alexanderbez]

Indeed @mossid. I would consider this a bug, but trivial to fix and test! Thanks 😄