Report durations

[rbtcollins]

Show the time spent doing downloads

It can be useful to know how long it takes to do a download, and
similarly while we use this to track installs, to do installs.

Looking at this I identified that if no data is received for a second
the current code overestimates the download performance (the missing)
seconds are elided and not counted towards averages. I've made a note
but fixing it is a different, larger change.

[rbtcollins]

This is a follow on branch to #1836 - the output looks like

target\debug\rustup.exe install nightly
info: syncing channel updates for 'nightly-x86_64-pc-windows-msvc'
info: latest update on 2019-05-07, rust version 1.36.0-nightly (a19cf18c7 2019-05-06)
info: downloading component 'rustc'
 60.7 MiB /  60.7 MiB (100 %)   2.8 MiB/s in  23 s ETA:   0 s
info: downloading component 'rust-std'
 55.2 MiB /  55.2 MiB (100 %)   2.7 MiB/s in  21 s ETA:   0 s
info: downloading component 'cargo'
  2.9 MiB /   2.9 MiB (100 %)   2.7 MiB/s in   1 s ETA:   0 s
info: downloading component 'rust-docs'
 11.0 MiB /  11.0 MiB (100 %)   2.8 MiB/s in   4 s ETA:   0 s
info: installing component 'rustc'
 60.7 MiB /  60.7 MiB (100 %)   6.2 MiB/s in  10 s ETA:   0 s
info: installing component 'rust-std'
 55.2 MiB /  55.2 MiB (100 %)   7.5 MiB/s in   8 s ETA:   0 s
info: installing component 'cargo'
info: installing component 'rust-docs'
 11.0 MiB /  11.0 MiB (100 %) 294.4 KiB/s in  37 s ETA:   0 s

  nightly-x86_64-pc-windows-msvc installed - rustc 1.36.0-nightly (a19cf18c7 2019-05-06)

info: checking for self-updates

[kinnison]

I'm okay with the second commit here (the meat of this PR) but we need to wait until the base PR is merged before we proceed. As that has a query on it, I'm not approving this yet.

[bors]

☔ The latest upstream changes (presumably #1836) made this pull request unmergeable. Please resolve the merge conflicts.