Returns {@code true} if all worker threads are currently idle.
An idle worker is one that cannot obtain a task to execute
because none are available to steal from other threads, and
there are no pending submissions to the pool. This method is
conservative; it might not return {@code true} immediately upon
idleness of all threads, but will eventually become true if
threads remain inactive.
@return {@code true} if all threads are currently idle
Returns {@code true} if all worker threads are currently idle. An idle worker is one that cannot obtain a task to execute because none are available to steal from other threads, and there are no pending submissions to the pool. This method is conservative; it might not return {@code true} immediately upon idleness of all threads, but will eventually become true if threads remain inactive.
@return {@code true} if all threads are currently idle