Make registry fully thread safe

Without this change potential incrementation can "go away" and can actually mismatch by 1 if run in multiple threads the same time. This can lead to super weird errors where counter is not as expected (been there, took me ages to debug).

ref: ruby-concurrency/concurrent-ruby#970

lib/rom/factory/sequences.rb

@@ -29,7 +29,7 @@ def next(key)
 
       # @api private
       def reset
-        @registry = Concurrent::Map.new { |h, k| h[k] = 0 }
+        @registry = Concurrent::Map.new { |h, k| h.compute_if_absent(k) { 0 } }
         self
       end
     end