Always run a full GC on rabbit_event

When a node has to process events generated by creating 100k
connections, 100k channels & 50k queues, it was observed that this
process would use up to 13GB of memory and never release it.

Since we are running a full GC whenever GC runs, this will slow the
process down by ~10%, but the memory usage is stable & all memory gets
eventually released. On a busy system, this can amount to many GBs.

Partner-in-crime: @essen

Author: gerhard

src/rabbit_event.erl

@@ -65,7 +65,7 @@
 %%----------------------------------------------------------------------------
 
 start_link() ->
-    gen_event:start_link({local, ?MODULE}).
+    gen_event:start_link({local, ?MODULE}, [{spawn_opt, [{fullsweep_after, 0}]}]).
 
 %% The idea is, for each stat-emitting object:
 %%