embed_links: Take a lock on the message object while editing.
embed_links: Take a lock on the message object while editing.

We leave the fetching of links outside of the lock, as they could take
seconds, which is an unreasonable amount of time to hold a lock on the
message row.  This may result in unnecessary work, in the case that
the message was since edited, but the unnecessary work is preferable
to blocking other work on the message row for the duration.

andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,zulip/zulip,zulip/zulip,andersk/zulip,zulip/zulip