Retrieves and removes the next watch key, waiting if necessary up to the specified wait time if none are yet present.

Parameters:

timeout		how to wait before giving up, in units of unit
unit		a TimeUnit determining how to interpret the timeout parameter

Returns:  the next watch key, or null

Exceptions:

ClosedWatchServiceException		if this watch service is closed, or it is closed while waiting for the next key
InterruptedException		if interrupted while waiting