The previous commit was broken.  D'oh!

The IsActive() method returned true even if the timer was not active,
after it completed once.  This broke the state file timer, and the
state file was not saved periodically.

This was broken by commit 84d20d9e, which deleted the "!" from the
check.

Turns out the lock-free code using atomics was not thread-safe.  The
given callback could be invoked by GLib before the source_id attribute
was assigned.  This commit changes the DeferredMonitor class to use a
Mutex to block the event loop until source_id is assigned.  This bug
does not exist in the 0.19 branch because it does not use the GLib
main loop anymore.

Fixes hanging child process in the "pipe" output plugin.

We have a better solution now.

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.

.. and rename it to Flush().

First check if an IPv6 socket can be created.

Regression from previous commit.  D'oh!

Use a std::list which can be appended at the end.

Eliminate the global variable "shutdown_loop".

Remove the const_cast from HttpdClient.cxx, and avoid one allocation
in ClientRead.cxx.

The header glib_compat.h is now obsolete and can be removed.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Reduces header dependencies.

Replaces GLib's GError.

Implement an event loop without GLib.

Fixes regression from commit 018f4155.

Prepare for migrating away from GLib.

Unsigned and portable.

Handle timeout internally.

Simplify the API, don't use GLib specific integer type.

Remove a GLib dependencies from class EventLoop and DatabaseLock.

Replaces io_thread_call().  This approach is more generic and easier
to use due to std::function.

Track which thread runs the EventLoop and provide a check whether
we're currently inside.

Prepare for adding more code.

Merge with CommitEventFlags().