Example of using CockroachDB with diesel

This is almost verbatim the example from

https://github.com/diesel-rs/diesel/tree/3ae353c3aff8a7ea64ed6cb39b3a045ac86cd60e/examples/postgres

with minor adjustments:

1. after diesel setup, edit the created up.sql and down.sql to contain only a trivial statement (like SELECT 1). This is necessary because by default they contain a random trigger that CockroachDB can't handle.
2. schema.rs was manually spelled out using the table! macro. The tutorial uses infer_schema! for which we don't have all the internals. Note that we use BigInt to properly support CockroachDB's SERIAL type. In turn, struct Post has id: i64 instead of i32.
3. Some adjustments in show_posts to list the ID which is otherwise impossible to guess.
4. Pinned the diesel dependency for no good reason (to hopefully have this go stale later).

There are likely more problems not discovered by this toy example. See the tracking issue.