Enforce Fleetspeak message size limit

Fleetspeak has a [message size limit of 2 MiB][1]. By not enforcing this limit in the connector library, we delay the issue and make applications end in cryptic way (as Fleetspeak terminates the connection).

[1]: https://github.com/google/fleetspeak/blob/126fd45b8732cb0998a5a93845489887daf89bd9/fleetspeak/src/client/channel/channel.go#L50