Merge pull request #150 from authorNari/protect_insert_from_competition

Output.insert should be exclueded
Revert "Merge pull request #150 from authorNari/protect_insert_from_competition"

This reverts commit 161fc9019c91a045c14dddf7a1d12f92ca14c57b, reversing
changes made to dc7178cbde06aed26dd00c62e5c37acbdd50e940.
$stdout is a shared data of all threads, so we must exclude other threads when it modify.
Since multi-threads may call Output.insert at same time (e.g. Core.error), it may causes a race condition.
doesn't swap $stdout in recusive call.