Problems found by "make distcheck".

Fix for the problem where order with priorities gets out of whack in case it's
reordered by SetRandom() while another song is currently playing.
What happens is, if some song is already playing and you have set some
priorities before switching on the random mode, and then turn the mode on, the
original code swaps position of the first song in the order (i.e., the highest
priority song) with current, so that current is 0 (which it should be). The
problem is, the "original" first song then goes to the place "current" song was
after reordering, wherever that is, instead of going after the "current" song.
This patch fixes the issue.
Also the fix makes MoveOrder() public, because why shouldn't it be, anyway.  It
certainly makes more sense than just having SwapOrders() public for some
reason.
Signed-off-by: Eugene Baklanov <miltenfiremage@gmail.com>

Forward the configured ReplayGainMode to all outputs and decoders.

The CurlRequest gives us a more low-level API to CURL without the
InputStream interface, integrated into our IOThread.

Simplifies the code and reduces the number of IOThread roundtrips.

Fixes assert failure after unpausing.

Make sure this deprecated API is not used.

We need purely RAII-style, and this is std::lock_guard.

Reduce overhead by eliminating MultipleOutputs::ResetReopen().