only enter IdleState if necessary in idleIfNecessary

There's no need to enter IdleState (and incur synchronization
overhead) unless another thread is waiting to enter ExclusiveState.
This change improves the performance of the MemoryRamp test by a
factor of about 100.
Merge pull request #278 from dicej/idleifnecessary-performance

only enter IdleState if necessary in idleIfNecessary