feat: handle multiple http servers with one graceful service

[sc-zenokerr]

Prerequisite to fix sand issue: Scalingo/sand#271 (since graceful v1.1.2)
Related to PR #909
Related to Story STORY-828

• Add a changelog entry in CHANGELOG.md

[leo-scalingo]

A small comment:

Do we really have to define the number of servers in the constructor as option. Can't it be counted when s.ListenAndServe is called?

[sc-zenokerr]

We need to know "when" to call the tableflip upgrader "Ready()" method.

I saw three options:

1. Add a method to the API to tell graceful that all listeners have been added
2. Add a config option to let graceful know in advance how many httpServers to expect
3. Add a short timeout so that all httpServers must be registered before the timeout

I chose option 2 because:

• minimal change to the API
• it is quite clear

I am happy to change it if you have any better ideas / preferences.

In the meantime, there is a "race" in this PR, so I am going to fix that, and send an update shortly.

[leo-scalingo]

We need to know "when" to call the tableflip upgrader "Ready()" method.

I saw three options:

1. Add a method to the API to tell graceful that all listeners have been added

2. Add a config option to let graceful know in advance how many httpServers to expect

3. Add a short timeout so that all httpServers must be registered before the timeout

I chose option 2 because:

* minimal change to the API

* it is quite clear

I am happy to change it if you have any better ideas / preferences.

In the meantime, there is a "race" in this PR, so I am going to fix that, and send an update shortly.

Really good explanation, LGTM

[leo-scalingo]

Validating the chosen methodology, letting Pierre for the code analysis :)

[sc-zenokerr]

@curzolapierre this is ready for code review now.

[curzolapierre]

Just some comments about style of logs and errors.

Otherwise LGTM

[sc-zenokerr]

@EtienneM Before I merge this, I would appreciate a discussion about it if you have time. Thanks in advance