support/db: upgrade github.com/go-sql-driver/mysql from 2e00b5cd7039 to v1.4.0

[leighmcculloch]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

Summary

Upgrade dependency github.com/go-sql-driver/mysql from 2e00b5cd7039 (Dec
24, 2016), which is a couple commits after v1.3.0, to v1.4.0 (Jun 3rd,
2018).

Goal and scope

This is part of making the transition to Modules (#1634).

The mysql module is used directly by this repositories support/db
package. The mysql module is also used indirectly via the
github.com/jmoiron/sqlx module which we have pinned to v1.2.0. We
use the sqlx package in services/ticker, services/horizon and
support/db. The sqlx package's tests have a dependency on mysql
v1.4.0 and even though it is only their tests, Modules carries that
minimum requirement to our project, even though we are not dependent on
the tests.

                 +-----------------------------+
                 | +------+      +-----------+ |
      +----------->+ sqlx +<-----+ sqlx.test | |
      |   v1.2.0 | +------+      +-----------+ |
      |          +-----------------------------+
+-----+------+                         |
| stellar/go |                         |
+-----+------+                         |
      |                                |
      |            +-------+           |
      +----------->+ mysql +<----------+
                   +-------+ v1.4.0

This isn't ideal for us because we don't particularly care about
dependencies of our dependencies tests, but the perspective of the Go
team has been that a dependencies test dependencies are a signal to
compatability of the module in general. e.g. If the sqlx devs say that
to test sqlx they need at least mysql v1.4.0, there's a good chance sqlx
is only compatible with v1.4.0+.

I considered two approaches to resolve this.

1. Upgrade mysql to v1.4.0, which is this change.
2. Downgrade sqlx two commits to drop it's go.mod file. The last two
   commits on the version we have are only the addition of a go.mod file.
   Without it we can pin the version of mysql that we have today. This
   isn't a good long term plan though, as sooner or later we're going to
   need to be bumped.

I've gone with this change because it better embraces the best practices
from the perspective of the core Go developers and it will probably
cause us less problems long term. The diff in the mysql dependency is 69
commits, 38 changed files, 4,928 additions, 797 deletions, 12 new
features, 12 bug fixes, and numerous other changes. It's not
insignificant and is probably not possible for one of us to review and
be able to meaningful identify if upgrading would be harmful. The best
thing we can do is rely on our tests here.

Summary of changes

• Update mysql package to v1.4.0.
• Include addition of appengine package that was added automatically as it's a dependency of the mysql package and was missing from the .lock file. (This should be meaningless to us, unless we were to deploy to Google AppEngine.)

Known limitations & issues

N/A

What shouldn't be reviewed

N/A

[ire-and-curses]

1.4 Changelog is here: https://github.com/go-sql-driver/mysql/blob/master/CHANGELOG.md#version-14-2018-06-03

It doesn't look like anything too major to me. Given that the dependency for sqlx is only in the tests, and the changes are mostly not breaking, I think we should try it out.

[leighmcculloch]

@ire-and-curses I want to make sure this isn't missed. The requirement to use v1.4.0 is only from the sqlx tests, but we do use the mysql dependency, so the dependency changing here will change the code we run in production. I've just re-requested your approval because I wanted to make sure this was communicated and I hadn't miscommunicated about this in the description.

[ire-and-curses]

Understood that we use it directly in support/db - thanks for pointing that out. Let's proceed.