The gw_time defines the RX time by the gateway, the ns_time defines when
it was received by the NS. The latter could for example help to debug
latency between the GW <> NS.
This reduces the amount of dependencies in case not all features are
being used. E.g. tonic is only needed if using gRPC and pbjson,
pbjson-types and serde are only needed if using the JSON serialization.
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.
