populate_db: Extract/fix mark_all_messages_as_read.
populate_db: Extract/fix mark_all_messages_as_read.

The prior version clobbered all flags, which means
we had unrealistic values for is_private.

Now we only touch the unread flag, which
also means when we go next to create alert words,
those will now work.

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