common: refactor health into its own lib crate

[xvello]

As more services will be added to the repo, we should have a story around code sharing. This PR uses the health module as a proposition for that code sharing topology. Let's:

• manage a set of small crates instead of one big common crate: should help avoid side-effects and keep compilation parallelism higher
• keep the crates un-versioned and un-published for now, imported with a path reference
• keep the crate names short and simple, but I'm open to adding a ph- prefix all internal crate names to easily differentiate them

I also sorted the Cargo.toml files while I was shuffling stuff around. Overall, this should be a no-op PR.

[tomasfarias]

I like it!

I don't think we need the ph prefix for now, as all crates would be under common/, but maybe if in the future we end up pulling an external crate with a name conflict.