update_message_backend: Fix memcached queries in loop.
update_message_backend: Fix memcached queries in loop.

This fixes a bug where update_message_backend would do one memcached
query per user receiving a given message.  Right now we just do a
single bulk database query, but in principle we could use
generic_bulk_cached_fetch to use the cache as well.

PhilSk/zulip,jphilipsen05/zulip,rht/zulip,andersk/zulip,jainayush975/zulip,samatdav/zulip,dattatreya303/zulip,mahim97/zulip,christi3k/zulip,showell/zulip,showell/zulip,punchagan/zulip,kou/zulip,shubhamdhama/zulip,showell/zulip,amanharitsh123/zulip,sonali0901/zulip,SmartPeople/zulip,jrowan/zulip,andersk/zulip,punchagan/zulip,rht/zulip,brainwane/zulip,eeshangarg/zulip,tommyip/zulip,timabbott/zulip,tommyip/zulip,dhcrzf/zulip,shubhamdhama/zulip,timabbott/zulip,ryanbackman/zulip,aakash-cr7/zulip,souravbadami/zulip,vabs22/zulip,zulip/zulip,vaidap/zulip,brockwhittaker/zulip,andersk/zulip,ryanbackman/zulip,vabs22/zulip,Galexrt/zulip,verma-varsha/zulip,synicalsyntax/zulip,synicalsyntax/zulip,zulip/zulip,brainwane/zulip,Galexrt/zulip,andersk/zulip,brainwane/zulip,souravbadami/zulip,tommyip/zulip,aakash-cr7/zulip,punchagan/zulip,PhilSk/zulip,blaze225/zulip,SmartPeople/zulip,Galexrt/zulip,dattatreya303/zulip,showell/zulip,eeshangarg/zulip,jackrzhang/zulip,vabs22/zulip,dawran6/zulip,rht/zulip,aakash-cr7/zulip,rht/zulip,Galexrt/zulip,timabbott/zulip,blaze225/zulip,dhcrzf/zulip,jrowan/zulip,JPJPJPOPOP/zulip,susansls/zulip,hackerkid/zulip,kou/zulip,vaidap/zulip,samatdav/zulip,brockwhittaker/zulip,SmartPeople/zulip,vabs22/zulip,amanharitsh123/zulip,SmartPeople/zulip,andersk/zulip,dawran6/zulip,j831/zulip,shubhamdhama/zulip,eeshangarg/zulip,synicalsyntax/zulip,timabbott/zulip,susansls/zulip,showell/zulip,showell/zulip,christi3k/zulip,sharmaeklavya2/zulip,dattatreya303/zulip,brainwane/zulip,blaze225/zulip,JPJPJPOPOP/zulip,j831/zulip,susansls/zulip,susansls/zulip,sonali0901/zulip,hackerkid/zulip,dattatreya303/zulip,tommyip/zulip,brockwhittaker/zulip,rishig/zulip,ryanbackman/zulip,synicalsyntax/zulip,amanharitsh123/zulip,verma-varsha/zulip,kou/zulip,mahim97/zulip,JPJPJPOPOP/zulip,jrowan/zulip,verma-varsha/zulip,dhcrzf/zulip,PhilSk/zulip,jainayush975/zulip,tommyip/zulip,aakash-cr7/zulip,sonali0901/zulip,punchagan/zulip,verma-varsha/zulip,rishig/zulip,JPJPJPOPOP/zulip,j831/zulip,ryanbackman/zulip,showell/zulip,vabs22/zulip,kou/zulip,christi3k/zulip,synicalsyntax/zulip,mahim97/zulip,sonali0901/zulip,jrowan/zulip,jphilipsen05/zulip,sonali0901/zulip,vaidap/zulip,sharmaeklavya2/zulip,andersk/zulip,aakash-cr7/zulip,shubhamdhama/zulip,synicalsyntax/zulip,brockwhittaker/zulip,vaidap/zulip,tommyip/zulip,jphilipsen05/zulip,brockwhittaker/zulip,rishig/zulip,souravbadami/zulip,sharmaeklavya2/zulip,susansls/zulip,amanharitsh123/zulip,dhcrzf/zulip,SmartPeople/zulip,j831/zulip,samatdav/zulip,j831/zulip,Galexrt/zulip,samatdav/zulip,samatdav/zulip,jphilipsen05/zulip,jrowan/zulip,amanharitsh123/zulip,rht/zulip,hackerkid/zulip,souravbadami/zulip,dawran6/zulip,eeshangarg/zulip,Galexrt/zulip,jainayush975/zulip,souravbadami/zulip,christi3k/zulip,sonali0901/zulip,dattatreya303/zulip,vaidap/zulip,blaze225/zulip,vaidap/zulip,dawran6/zulip,rishig/zulip,dattatreya303/zulip,timabbott/zulip,zulip/zulip,jphilipsen05/zulip,susansls/zulip,punchagan/zulip,PhilSk/zulip,Galexrt/zulip,blaze225/zulip,hackerkid/zulip,PhilSk/zulip,sharmaeklavya2/zulip,souravbadami/zulip,brockwhittaker/zulip,dhcrzf/zulip,jackrzhang/zulip,kou/zulip,jackrzhang/zulip,verma-varsha/zulip,hackerkid/zulip,jainayush975/zulip,rishig/zulip,jackrzhang/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,verma-varsha/zulip,zulip/zulip,sharmaeklavya2/zulip,hackerkid/zulip,timabbott/zulip,j831/zulip,jainayush975/zulip,dhcrzf/zulip,rht/zulip,synicalsyntax/zulip,dawran6/zulip,eeshangarg/zulip,JPJPJPOPOP/zulip,christi3k/zulip,shubhamdhama/zulip,rht/zulip,christi3k/zulip,brainwane/zulip,jackrzhang/zulip,JPJPJPOPOP/zulip,dhcrzf/zulip,kou/zulip,andersk/zulip,tommyip/zulip,sharmaeklavya2/zulip,mahim97/zulip,mahim97/zulip,ryanbackman/zulip,aakash-cr7/zulip,zulip/zulip,vabs22/zulip,timabbott/zulip,rishig/zulip,samatdav/zulip,eeshangarg/zulip,rishig/zulip,jphilipsen05/zulip,SmartPeople/zulip,amanharitsh123/zulip,jackrzhang/zulip,dawran6/zulip,punchagan/zulip,shubhamdhama/zulip,brainwane/zulip,hackerkid/zulip,ryanbackman/zulip,shubhamdhama/zulip,brainwane/zulip,jrowan/zulip,PhilSk/zulip,mahim97/zulip,jackrzhang/zulip,jainayush975/zulip,punchagan/zulip,zulip/zulip,blaze225/zulip