refactor: Extract message_fetch.get_backfill_anchor().
refactor: Extract message_fetch.get_backfill_anchor().

This is just moving code.

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