This makes it possible to add gateways to a multicast-group, which in
case configured will always be used for transmitting the multicast
downlinks.
This also moves the multicast class-c scheduling to the multicast-group
configuration. Options are delay between multiple gateways, or GPS time
synchronized transmission.
This includes:
* Changing the modulation parameters to its own type.
* Changing the timing parameters to its own type.
* Change the gateway_id to string. As the json encoding for bytes fields
is base64, this was confusing some users.
* Change the uplink / downlink id to uint32 from uuid. A string
representation of the UUID field (for the same reason as the gateway
id) would consome quite some additional bytes. An uint32 provides
sufficient uniqueness for the purpose of uplink / downlink.
