Revert "Use default work pool for definition import"

This reverts commit 1a60b43.

Author: michaelklishin

src/rabbit_definitions.erl

@@ -350,6 +350,7 @@ do_concurrent_for_all(List, WorkPoolFun) ->
          %% keys are expected to be atoms
          ok = gatherer:fork(Gatherer),
          worker_pool:submit_async(
+           ?IMPORT_WORK_POOL,
            fun() ->
                    try
                        WorkPoolFun(M)