Fixed race condition in onSelected().

WebSocket and multiplexed protocols are always read interested.
It may happen that while the application is writing, the write
blocks, resulting in a call to changeInterests().
At the same time, the selector may detect data to read and call
onSelected(), so there is a possibility that onSelected() runs
concurrently with changeInterests().

The fix adds an additional state (PROCESSING) that isolates the
changes that onSelected() makes to _interestOps, spin-waiting if
changeInterests() is running concurrently.
Likewise, changeInterests() spin-waits until onSelected() is running
concurrently.