revert default value of flag folly_memory_idler_purge_arenas

Reviewed By: yfeldblum

Differential Revision: D27575037

fbshipit-source-id: fa5817ee3fe1eca99e83f26a13e43ec4a4a6e7cf

folly/detail/MemoryIdler.cpp

@@ -34,7 +34,7 @@
 
 DEFINE_bool(
     folly_memory_idler_purge_arenas,
-    false,
+    true,
     "if enabled, folly memory-idler purges jemalloc arenas on thread idle");
 
 namespace folly {