Make default worker pool size configurable

Part of rabbitmq/rabbitmq-server#2030.

priv/schema/rabbit.schema

@@ -391,6 +391,13 @@ end}.
     {datatype, integer}
 ]}.
 
+%% Default worker process pool size. Used to limit maximum concurrency rate
+%% of certain operations, e.g. queue initialisation and recovery on node boot.
+
+{mapping, "default_worker_pool_size", "rabbit.default_worker_pool_size", [
+    {datatype, integer}, {validators, ["non_negative_integer"]}
+]}.
+
 %% Password hashing implementation. Will only affect newly
 %% created users. To recalculate hash for an existing user
 %% it's necessary to update her password.

test/config_schema_SUITE_data/rabbit.snippets

@@ -566,6 +566,14 @@ credential_validator.regexp = ^abc\\d+",
   [{rabbit,[{log, [{categories, [{connection, [{file, "file_name_connection"}]},
                                  {channel, [{file, "file_name_channel"}]}]}]}]}],
   []},
+
+ {default_worker_pool_size,
+  "default_worker_pool_size = 512",
+  [{rabbit, [
+      {default_worker_pool_size, 512}
+    ]}],
+  []},
+
  {delegate_count,
   "delegate_count = 64",
   [{rabbit, [