Fix usage of threads for pages migration

The main thread already is one that is `running`.
The child threads should always wrap only operation
instead of the whole thread. This ensures that wrapping
operation can hold a lock when loading code.

In a worst case scenario it will result that initially till
all threads do load all relevant code it will be run effectively
in sequence. This will resolve once all dependencies get loaded
lazilly.

app/services/pages/migrate_from_legacy_storage_service.rb

@@ -24,9 +24,7 @@ module Pages
       @queue.close
 
       @logger.info("Waiting for threads to finish...")
-      ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
-        threads.each(&:join)
-      end
+      threads.each(&:join)
 
       { migrated: @migrated, errored: @errored }
     end
@@ -34,8 +32,8 @@ module Pages
     def start_migration_threads
       Array.new(@migration_threads) do
         Thread.new do
-          Rails.application.executor.wrap do
-            while batch = @queue.pop
+          while batch = @queue.pop
+            Rails.application.executor.wrap do
               process_batch(batch)
             end
           end
@@ -51,6 +49,11 @@ module Pages
       end
 
       @logger.info("#{@migrated} projects are migrated successfully, #{@errored} projects failed to be migrated")
+    rescue => e
+      # This method should never raise exception otherwise all threads might be killed
+      # and this will result in queue starving (and deadlock)
+      Gitlab::ErrorTracking.track_exception(e)
+      @logger.error("failed processing a batch: #{e.message}")
     end
 
     def migrate_project(project)