Order by concurrency_key before distinct

Microsoft SQL Server requires a deterministic order when using `limit` (aka you have to provide `ORDER BY`). By default, in the MSSQL adapter, we inject a `ORDER BY [primary_key]` clause to achieve so, if not any `order` has been specified in the query.

This now leads to an issue in Solid Queue, where `concurrency_key` is selected and MSSQL complains that `id` is not in its `SELECT` clause.

I first looked into fixing this in the SQL adapter, but then found a test in `activerecord` itself (`test_pluck_and_distinct`) that orders first before calling `distinct`. So I would suggest to align Solid Queue here and order by `concurrency_key` prior to calling `distinct`.

I am aware that the existing code works with SQlite, MySQL and PostgreSQL as these do not require to pass an `ORDER BY` clause with `LIMIT`. I don't think this small addition will cause any troubles on the other DBMS systems.

Author: andyundso

app/models/solid_queue/blocked_execution.rb

@@ -12,7 +12,7 @@ class BlockedExecution < Execution
     class << self
       def unblock(limit)
         SolidQueue.instrument(:release_many_blocked, limit: limit) do |payload|
-          expired.distinct.limit(limit).pluck(:concurrency_key).then do |concurrency_keys|
+          expired.order(:concurrency_key).distinct.limit(limit).pluck(:concurrency_key).then do |concurrency_keys|
             payload[:size] = release_many releasable(concurrency_keys)
           end
         end