switch to blessed mapstructure fork in reva

https://github.com/go-viper/mapstructure is the blessed fork of https://github.com/mitchellh/mapstructure we should update

also ... the package has custom decoders that can be used to decode eg. [time.Duration](https://github.com/go-viper/mapstructure/blob/c97971d2ae39a0b7498b19fdd652c01af391b13c/decode_hooks.go#L161) it just needs to be added when constructing the Decoder: https://github.com/go-viper/mapstructure/blob/c97971d2ae39a0b7498b19fdd652c01af391b13c/mapstructure_examples_test.go#L342-L346