Introduced factory method to create JDK's Selector instances.
Fixed race condition causing onSelected() to throw IllegalStateException.

The race was happening when updateKey() lost the race with
changeInterests() to update the interests and subsequently the key.
In that case, the selector thread that called updateKey() was returning
while changeInterests() was executing the CHANGING state.
Since updateKey() lost the race, the actual key interests was still
(typically) OP_READ so the selector thread would select() and call
onSelected() while changeInterests() was still executing, causing the
IllegalStateException.