Record how long storage patches took to apply

Summary:
It's hard for us to predict how long patches and migrations will take in the general case since it varies a lot from install to install, but we can give installs some kind of rough heads up about longer patches. I'm planning to just put a sort of hint for things in the changelog, something like this:

To make this easier, start storing how long stuff took. I'll write a little script to dump this into a table for the changelog.

Test Plan:
Ran bin/storage status:

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14320