import: Avoid unnecessary forks when downloading attachments.
import: Avoid unnecessary forks when downloading attachments.

The previous implementation used run_parallel incorrectly, passing it
a set of very small jobs (each was to download a single file), which
meant that we'd end up forking once for every file to download.

This correct implementation sends each of N threads 1/N of the files
to download, which is more consistent with the goal of distributing
the download work between N threads.

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