performance: Optimize max_message_id calculation.
performance: Optimize max_message_id calculation.

We calculate `max_message_id` for the mobile client.

Our query now no longer joins to the Message table
and just grabs one value instead of fat objects.

shubhamdhama/zulip,zulip/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,showell/zulip,brainwane/zulip,synicalsyntax/zulip,brainwane/zulip,hackerkid/zulip,kou/zulip,punchagan/zulip,shubhamdhama/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,kou/zulip,kou/zulip,showell/zulip,zulip/zulip,brainwane/zulip,showell/zulip,zulip/zulip,kou/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,rht/zulip,rht/zulip,showell/zulip,timabbott/zulip,zulip/zulip,punchagan/zulip,synicalsyntax/zulip,showell/zulip,brainwane/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,timabbott/zulip,timabbott/zulip,rht/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,timabbott/zulip,rht/zulip,shubhamdhama/zulip,showell/zulip,synicalsyntax/zulip,timabbott/zulip,zulip/zulip,shubhamdhama/zulip,zulip/zulip,synicalsyntax/zulip,andersk/zulip,zulip/zulip,shubhamdhama/zulip,eeshangarg/zulip,brainwane/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,shubhamdhama/zulip,timabbott/zulip,brainwane/zulip,synicalsyntax/zulip,showell/zulip,timabbott/zulip,shubhamdhama/zulip,andersk/zulip,synicalsyntax/zulip,hackerkid/zulip