`Appender#stop` is not idempotent

## Description

In Log4j 2 managers are reference counted: if you have two `FileAppender`s writing to the same file, there will be a single `FileManager` with a reference count of 2.

Calling `stop()` on a stopped appender **always** calls `stop()` on the underlying manager. In the situation described above it is effectively possible to stop both file appender by calling `stop()` twice on one of them.

This a problem, because those appenders might not even be in the same configuration or attached to the same logger context.

## Configuration

**Version:** up to 2.20.0 and 3.0.0-alpha1

## Reproduction

Call `stop()` multiple times on a file appender. All other appenders pointing to the same file will stop logging.