sent_messages: Pass correct reference of this in setTimeout.
sent_messages: Pass correct reference of this in setTimeout.

We explicitly bind `this` to MessageState class which otherwise
was defaulting to `window`.

This resulted in variables like `this.received` and `this.local_id`
being incorrectly interpreted by called function
as `window.(received | local_id)` which are `undefined`.
Hence, frontend thinks that the message was never received.

It was noticed since this was the common log message when
a double message send bug was observed. This change in no
was indicates fixing of the double send bug, but is hopefully
one step forward in that direction.

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