refactor: Extract send_subscription_add_events.
refactor: Extract send_subscription_add_events.

This function now does all the work that we used
to do with notify_subscriptions_added happening
inside a loop.

There's a small fine-tuning here, where we only
get recent traffic on streams that we're actually
sending events for.

punchagan/zulip,kou/zulip,punchagan/zulip,eeshangarg/zulip,eeshangarg/zulip,showell/zulip,zulip/zulip,punchagan/zulip,rht/zulip,showell/zulip,andersk/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,showell/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,andersk/zulip,hackerkid/zulip,showell/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,kou/zulip,kou/zulip,rht/zulip,punchagan/zulip,rht/zulip,showell/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,showell/zulip,kou/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip