Add support for CHATHISTORY TARGETS

The main motivation is to avoid missing direct messages coming from
other users.

A nice side-effect is that we no longer need to issue CHATHISTORY
queries for each channel we JOIN: instead, we can only fetch
history for targets known to have new messages available (as indicated
by CHATHISTORY TARGETS).

We use read receipts instead of delivery receipts, so that reloading
the webapp restores the exact same state (ie, unread messages are
re-fetched).

References: https://github.com/ircv3/ircv3-specifications/pull/450

lib/client.js

@@ -575,6 +575,24 @@ export default class Client extends EventTarget {
 		});
 	}
 
+	fetchHistoryTargets(t1, t2) {
+		var msg = {
+			command: "CHATHISTORY",
+			params: ["TARGETS", "timestamp=" + t1, "timestamp=" + t2, 1000],
+		};
+		return this.fetchBatch(msg, "draft/chathistory-targets").then((batch) => {
+			return batch.messages.map((msg) => {
+				if (msg.command != "CHATHISTORY" || msg.params[0] != "TARGETS") {
+					throw new Error("Cannot fetch chat history targets: unexpected message " + msg);
+				}
+				return {
+					name: msg.params[1],
+					latestMessage: msg.params[2],
+				};
+			});
+		});
+	}
+
 	listBouncerNetworks() {
 		if (!this.enabledCaps["soju.im/bouncer-networks"]) {
 			return Promise.reject(new Error("Server doesn't support the BOUNCER extension"));