performance: Optimize send_subscription_add_events.
performance: Optimize send_subscription_add_events.

We avoid repeating the same calculations over and
over again for the same stream.

This helps, but the real bottleneck in this function
is that send_event usually takes at least a millisecond,
and that adds up quickly if you're doing something
like subscribing 5k users to a new stream.

eeshangarg/zulip,rht/zulip,andersk/zulip,kou/zulip,zulip/zulip,andersk/zulip,kou/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,rht/zulip,kou/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,kou/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip