Instead of using this as a base class implementing a virtual method,
the new class IdleEvent can be used as a variable, decoupling
IdleMonitor's internal state from the derived class.
This is similar to commit 30a5dd267b
which refactored TimeoutMonitor to TimerEvent.
Since we switched from autotools to Meson in commit
94592c1406, we don't need to include
`config.h` early to properly enable large file support. Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.
This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.
Postpone the write using IdleMonitor instead of scheduling a write
event. This reduces the number of system calls, because we don't need
to register and unregister the write event in epoll.
