Handle timeout internally.

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

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

Eliminate input_stream_init() and input_stream_deinit().

Hide the definition from C code, to prepare the transition to C++.

Fixes assertion failure (regression).

Using initializer list.

Not necessary, GLib will pick up the updated GPollFD automatically.

Use the "url" attribute from the base class instead.

g_source_get_current_time() is deprecated since GLib 2.28.  This patch
adds a compatibility wrapper for older GLib versions to glib_compat.h.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

To check for errors without reading.  The decoder thread wants to do
that, before it passes the input stream to the plugin.

Fix memory leak.

Pausing and resuming after every little chunk adds lots of overhead.
Add a lower level for resuming the stream.

curl_easy_pause(CURLPAUSE_CONT) may have added enough data to go into
pause again.

Call _update_fds() directly.  This is possible because it's only
called from within the I/O thread.

Fix deadlock.

The global data structures are now lock-free, because they are
accessed only from the I/O thread.  By using per-request locks, we
have finer grained locking, preparing for locks shared with the
client.

This adds some overheads for indirect calls to the I/O thread, but
reduces the amount of global locks.  Next step will be switching to
per-request locks.