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().