internal/dinosql: Implement advisory locks

[kyleconroy]

Fixes #211

[kyleconroy]

This still isn't perfect, mainly due to the void return type

$ sqlc generate
2019/12/22 19:12:29 unknown Postgres type: void
2019/12/22 19:12:29 unknown Postgres type: void
2019/12/22 19:12:29 unknown Postgres type: void

[kyleconroy]

Okay, it's looking good now

const acquireLock = `-- name: AcquireLock :exec
SELECT pg_advisory_xact_lock($1)
`

func (q *Queries) AcquireLock(ctx context.Context, key int64) error {
    _, err := q.db.ExecContext(ctx, acquireLock, key)                                                                                                                                                       
    return err
}