Switch to sqlx for compile-time checked queries

[jyn514]

A common source of outages is an incorrect query with no tests (#862, #829, #648, etc). While it's good to have tests for all queries, it's not always feasible (especially until #822 is fixed). SQLx offers that advantage that it can check all queries at build time without having to write a test for that specific query. It is also fully async (cc @Kixiron), although that might mean we have to wait until we switch to tokio 0.2.

To avoid needless database connections, this should only connect to the database when a query is changed or added, or a schema change is made: launchbadge/sqlx#249. During CI it should always connect to the database.

[pietroalbini]

During CI it should not connect to the database, or do a build with and without the database: we want to make sure the cached data is correct and a build can work without a connection.

[Nemo157]

By my understanding there will be a separate CLI utility that prepares the queries. So we can run a build job that calls sqlx-cli prepare or whatever the command is, then git diff --color --exit-code to show and fail if any file changed. Then the normal build can run without connecting to the database.

[Kixiron]

I think this’d be a great change, I’ll take a look at it once we’re async-capable

[jyn514]

I have a WIP for this in https://github.com/jyn514/docs.rs/tree/sqlx, using block_on since we're still stuck on iron.

[Nemo157]

(compare link: master...jyn514:sqlx)