Postgres connection

[elpiel]

Create a Postgres async client to be used in Sentry with tokio_postgres

[elpiel]

The problems I've encountered here were that:

• if we want a single client per thread, it's not possible to create it with thread_local as creating the client is asynchronous and thus we can't await!() inside thread_local
• other issues applies when I tried to use it with the async syntax, maybe because also of my lack of knowledge around futures, tokio and concurrency as a whole yet.
• couldn't find a way to use the Pool(r2d2_postgres) with hyper and the server future so we can have specific set of connections to use when connection comes

Current workaround was to implement it with the synchronous client and basically on each request make a new connection.

[elpiel]

PRs #5 and #6 both tackle this problem using bb8. It still looks quite ugly for the repository, but for now it's a start and it's using a Pool and it's async.

Further investigation should go into it, to see how we can make it easy for the Repositories to look and be more developer friendly.

[elpiel]

Some improvements, i.e. separation of concern, were done in PR #42 for implementing the ToSql & FromSql for Domain Structs, since this cannot be done outside of domain crate, we've created a wrapper tuple structs that will handle the conversion in the Postgres***Repository implementations

[elpiel]

closing in favour of #151