recent_senders: Use better variable names and comments.
recent_senders: Use better variable names and comments.

This makes the data structures in use here more readable.

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