This sets the default and allows customising how often we try to
download files from the API.
This does not affect `brew update` as we want to always check every time
on an explicit call.
Rather than failing every time we fail to update (breaking most usage
while offline) instead only fail if we cannot obtain the JSON and we
don't have a valid local file we can use.
Also tweak the timeout and retry logic and values to make a bit more
sense in this context and not be so noisy when offline.
- Use raw.githubusercontent.com to download cask source rather than
formulae.brew.sh. This allows us to remove these files
- output the tap's current `HEAD` for both formulae and cask JSON
- use this `HEAD` for the cask-source API to get the exact file on
raw.githubusercontent.com rather than just whatever is newest (which
is what the previous API did)
- set the `Tap` correctly when creating a `Cask` from the API
- if the `formula.json` file exists: print its modified time include
`brew config`
- memoize `tap.git_head` as we'll be calling it a lot in the same
process with the same value
