origin_client_ts to distinguish composed v. sent timestamps

[ara4n]

Suggestion
The intention of timestamp fields on events was always to distinguish timings clearly across the lifecycle of a message being sent: when the send button was hit (origin_client_ts); when it was received by the server (origin_server_ts); and possibly unsigned data for when it was delivered to the destination server (dest_server_ts); when it was received by a given destination client (dest_client_ts).

Historically, clients tended not to automatically flush their send queues, and so the user had to consciously interact when flushing the queue - meaning they could remove stale messages and psychologically were effectively "hitting send again" in order to send them. Therefore origin_server_ts was an adequate timestamp for the sent msgs.

Nowadays however, clients like matrix-rust-sdk automatically flush send queues in the background, meaning that clients send messages with arbitrary origin_server_ts which don't reflect when the user consciously intended a message to be sent. Therefore, we need to (at last) introduce origin_client_ts to distinguish the two, letting clients display timestamps based on when a message was created rather than when it happened to get received by the server.

See element-hq/element-meta#2678 for more details.