soft_deactivation: Make the stream_messages mutation logic clearer.
soft_deactivation: Make the stream_messages mutation logic clearer.

This changes our code from something that's pretty nasty bad behavior
mutating objects during a loop to only somewhat bad behavior.

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