Decrement the inflight counter on ConnectionRefused (#184)

src/db/pool.cr

@@ -120,8 +120,11 @@ module DB
           resource = if @idle.empty?
                        if can_increase_pool?
                          @inflight += 1
-                         r = unsync { build_resource }
-                         @inflight -= 1
+                         begin
+                           r = unsync { build_resource }
+                         ensure
+                           @inflight -= 1
+                         end
                          r
                        else
                          unsync { wait_for_available }