fix: close mysql connection after create database

[FabianKramm]

One problem we encountered when using Kine with the MySQL driver is that it uses an unexpected high amount of database connections, even after setting the flag --datastore-max-open-connections to a small number.

We investigated and found out that during the createDBIfNotExist function two database connections are created as well that are never closed. For the PostgreSQL driver the connection is closed as expected:

kine/pkg/drivers/pgsql/pgsql.go

Line 223 in 133503d

defer db.Close()

For the MySQL driver, these close after creation defers are missing and hence the connections stay open all the time. This PR adds two lines to close the database connections as expected after making sure the database exists in the MySQL server.

EDIT: Not sure why the drone ci build is failing, but it seems unrelated to this change

[brandond]

it looks like revive picked up some new nits. I can fix those tomorrow and you can rebase, or you're welcome to add a second commit that ignores the redefines-builtin-id check for now if you're up for it.

[FabianKramm]

@brandond thanks for the quick approve, I ignored the linter for the 2 functions, tests seem to run now

EDIT: tests passed 🎉