Optimize show_unreads --fix approach.
Optimize show_unreads --fix approach.

This now breaks the process of cleaning up unread counts for
non-active streams into a three step process.

This allows us to use our unread message flags index, at least
in testing on dev.  Here is the relevant excerpt from explain
analyze:

    Bitmap Index Scan on zerver_usermessage_unread_message_id

rishig/zulip,dhcrzf/zulip,Galexrt/zulip,punchagan/zulip,hackerkid/zulip,showell/zulip,punchagan/zulip,brainwane/zulip,brockwhittaker/zulip,shubhamdhama/zulip,shubhamdhama/zulip,rht/zulip,timabbott/zulip,amanharitsh123/zulip,dhcrzf/zulip,synicalsyntax/zulip,brockwhittaker/zulip,hackerkid/zulip,timabbott/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,synicalsyntax/zulip,kou/zulip,vaidap/zulip,synicalsyntax/zulip,jackrzhang/zulip,amanharitsh123/zulip,zulip/zulip,tommyip/zulip,timabbott/zulip,hackerkid/zulip,kou/zulip,dhcrzf/zulip,mahim97/zulip,verma-varsha/zulip,mahim97/zulip,mahim97/zulip,showell/zulip,eeshangarg/zulip,jackrzhang/zulip,showell/zulip,shubhamdhama/zulip,brainwane/zulip,brockwhittaker/zulip,vaidap/zulip,verma-varsha/zulip,Galexrt/zulip,tommyip/zulip,Galexrt/zulip,showell/zulip,brockwhittaker/zulip,rht/zulip,timabbott/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,andersk/zulip,amanharitsh123/zulip,rht/zulip,amanharitsh123/zulip,zulip/zulip,zulip/zulip,rishig/zulip,hackerkid/zulip,synicalsyntax/zulip,rht/zulip,verma-varsha/zulip,timabbott/zulip,brainwane/zulip,amanharitsh123/zulip,timabbott/zulip,brockwhittaker/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,shubhamdhama/zulip,tommyip/zulip,kou/zulip,rishig/zulip,kou/zulip,vaidap/zulip,punchagan/zulip,zulip/zulip,showell/zulip,shubhamdhama/zulip,Galexrt/zulip,brainwane/zulip,eeshangarg/zulip,synicalsyntax/zulip,vaidap/zulip,dhcrzf/zulip,dhcrzf/zulip,kou/zulip,mahim97/zulip,eeshangarg/zulip,synicalsyntax/zulip,rishig/zulip,mahim97/zulip,showell/zulip,Galexrt/zulip,rht/zulip,zulip/zulip,dhcrzf/zulip,Galexrt/zulip,brainwane/zulip,kou/zulip,brockwhittaker/zulip,hackerkid/zulip,vaidap/zulip,kou/zulip,verma-varsha/zulip,tommyip/zulip,timabbott/zulip,eeshangarg/zulip,punchagan/zulip,brainwane/zulip,rht/zulip,vaidap/zulip,jackrzhang/zulip,rishig/zulip,mahim97/zulip,jackrzhang/zulip,eeshangarg/zulip,shubhamdhama/zulip,tommyip/zulip,amanharitsh123/zulip,andersk/zulip,Galexrt/zulip,hackerkid/zulip,jackrzhang/zulip,synicalsyntax/zulip,verma-varsha/zulip,andersk/zulip,jackrzhang/zulip,jackrzhang/zulip,andersk/zulip,rishig/zulip,rishig/zulip,tommyip/zulip,verma-varsha/zulip,punchagan/zulip,dhcrzf/zulip,tommyip/zulip,punchagan/zulip,shubhamdhama/zulip,showell/zulip,brainwane/zulip