`FirstMessageTimestamp` enables members of the community to determine if
there are any messages they can fetch on the community channel(chat).
`FirstMessageTimestamp` is advertised by admin for each community chat
through `CommunityDescription`. It assumes admin is online frequently
enough to capture the first channel message.
For existing communities admin determines first message timestamp by
finding oldest chat message in its local database.
task: status-im/status-desktop#6731
65be6f2b96