Merge #6728

6728: Fallback to sequentially fetching specs on 429s r=indirect a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem is that sometimes `bundler` is unable to resolve certain gemfiles. Specifically, sometimes it does not respect the `required_ruby_version` setting. This causes some people to assume that `bundler` will always try to install the latest version of any dependency, regardless of the ruby version being run, because as a matter of fact, this feature sometimes just doesn't work. See for example the discussion at https://github.com/rspec/rspec/issues/25.

The problem was consistently reproducible until a few minutes ago with the following `Gemfile` under `ruby 2.3.7`

```ruby
source "https://rubygems.org"
ruby "2.3.7"
gem "berkshelf", "= 6.3.1"
```

```
$ docker run -it --rm --volume $(pwd):/app ruby:2.3.7 sh -c "cd /app && rm -f Gemfile.lock && bundle install"
Fetching gem metadata from https://rubygems.org/.............
Fetching gem metadata from https://rubygems.org/..
Resolving dependencies....
Fetching public_suffix 3.0.3
Installing public_suffix 3.0.3
Fetching addressable 2.5.2
Installing addressable 2.5.2
Fetching buff-extensions 2.0.0
Installing buff-extensions 2.0.0
Fetching hashie 3.6.0
Installing hashie 3.6.0
Fetching varia_model 0.6.0
Installing varia_model 0.6.0
Fetching buff-config 2.0.0
Installing buff-config 2.0.0
Using bundler 1.16.5
Fetching fuzzyurl 0.9.0
Installing fuzzyurl 0.9.0
Fetching tomlrb 1.2.7
Installing tomlrb 1.2.7
Fetching mixlib-config 2.2.13
Installing mixlib-config 2.2.13
Fetching mixlib-shellout 2.4.0
Installing mixlib-shellout 2.4.0
Fetching chef-config 14.5.33
Installing chef-config 14.5.33
Fetching libyajl2 1.2.0
Installing libyajl2 1.2.0 with native extensions
Fetching ffi-yajl 2.3.1
Installing ffi-yajl 2.3.1 with native extensions
Fetching mixlib-log 2.0.4
Installing mixlib-log 2.0.4
Fetching rack 2.0.5
Installing rack 2.0.5
Fetching uuidtools 2.1.5
Installing uuidtools 2.1.5
Fetching chef-zero 14.0.6
Installing chef-zero 14.0.6
Gem::RuntimeRequirementNotMetError: chef-zero requires Ruby version >= 2.4.0. The current ruby version is 2.3.0.
An error occurred while installing chef-zero (14.0.6), and Bundler cannot continue.
Make sure that `gem install chef-zero -v '14.0.6' --source 'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  berkshelf was resolved to 6.3.1, which depends on
    chef was resolved to 14.5.33, which depends on
      chef-zero
```

Funny enough, I can no longer reproduce it at the moment, I guess it depends on the specific load conditions of the rubygems.org servers?

### What was your diagnosis of the problem?

My diagnosis was that sometimes our resolution falls back to the old dependency API that didn't implement the `required_ruby_version` setting. In particular, this happens because the new API returns `Net::HTTPTooManyRequests`, so `bundler` gives up and defaults to the old API.

### What is your fix for the problem, implemented in this PR?

My fix is to, instead of directly fall back to the old API when rate limiting happens, try first to fetch the dependencies sequentially instead of in parallel still from the new API, so that rate limit does not affect us.

### Why did you choose this fix out of the possible options?

I chose this fix because it was the only idea that came up. As a matter of fact, #6471 and #6639 were closed because there was nothing we could do, so it seems like it's the only idea so far :)

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

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