This avoids the overhead of maintaining the selectedKeys() set

After the timer rewrite only the timer callbacks are executed
but stages are no longer automatically marked for execution. Timers that
send messages are not affected as that uses the standard activation mechanism.

This allows excluding those tests based on the pattern '^.*(?<!IT)$' in
IntelliJ

Unless we need to remove elements in the middle of a list, an array based variant is always faster.

There's no need for the current replica to further process the
request.