Keep the logging library out of this low-level libcurl wrapper.

We need at least GCC 8 anyway.

Fixes regression by commit 521e573b

Fixes regression by commit 7901b04c

Similar to commits 1686f4e8 and
30a5dd26

Fixes the !HAVE_THREADED_EVENT_LOOP build.

Not for MPD, but for other applications which might want to copy its
event loop, but do not need multi-threading.

Allows forward declaration.

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 30a5dd26
which refactored TimeoutMonitor to TimerEvent.

The new stable branch v0.22.x was forked off and is feature-frozen, so
the "master" branch will become the next major version eventually.

By making each SocketMonitor keep track of its ready flags, this
removes a lot of overhead from EventLoop::RemoveFD().

By bit-wise ANDing the reported flags with GetScheduledFlags(),
ERROR/HANGUP always get cleared.  This means the MPD event loop could
never report those conditions.