Adds registrator managed TTL and supports multiple checks in the consul backend

[pierredavidbelanger]

I know I am not the first to submit a PR for this.

I submit it anyways in case it can be useful.

Also, the reason I made this is first because I needed the TTL feature, then to learn a little about go (indeed those are my first lines of go code).

Do not hesitate to make me change things.

[pierredavidbelanger]

From what I can see, my past self just added a way to register multiple checks, one of each kind (so zero or one HTTP, zero or one TTL, etc..).

In retrospective I am not sure this was the right way to do this.

In fact, what I really needed from this PR was the managed TTL check.

Since then I have coded my own very limited Consul only clone of this tool that I use in production.