sds: add optional support for ephemeral messages

[adklempner]

Per the specifications https://rfc.vac.dev/vac/raw/sds/#ephemeral-messages :

Participants MAY choose to send short-lived messages for which no synchronization or reliability is required. These messages are termed ephemeral.

Ephemeral messages SHOULD be sent with lamport_timestamp, causal_history, and bloom_filter unset. Ephemeral messages SHOULD NOT be added to the unacknowledged outgoing buffer after broadcast. Upon reception, ephemeral messages SHOULD be delivered immediately without buffering for causal dependencies or including in the local log.

To support this, the class for a message channel must accept a boolean flag for when ephemeral messages are enabled.

If ephemeral messages are enabled, then a check should be performed on all sent and received messages to determine if they are ephemeral. If so, skip buffering and local log as described above.