api: re-generate protobuf code

Changes from ba6565ff6bf66e200842c7e6ab7112caa845d98a require to
re-generate protbuf related files.

Note that to register services using the methods generated by
`protoc-gen-go-grpc`, the service implementations must embed the
corresponding `Unimplemented<ServiceName>Server` for future
compatibility.  This is a behavior change from the grpc code generator
previously included with `protoc-gen-go`. One could optionally restore
the old behavior by passing the option
`requireUnimplementedServers=false` when generating files but this is
not recommended.

The 3 structs which implement the observer and peer services now embed
the respective `Unimplemented<ServiceName>Server` structs.

Signed-off-by: Robin Hahling <robin.hahling@gw-computing.net>