Fix Brotli decompression in 'nix log'

This didn't work anymore since decompression was only done in the
non-coroutine case.

Decompressors are now sinks, just like compressors.

Also fixed a bug in bzip2 API handling (we have to handle BZ_RUN_OK
rather than BZ_OK), which we didn't notice because there was a missing
'throw':

  if (ret != BZ_OK)
      CompressionError("error while compressing bzip2 file");

src/libstore/download.hh

@@ -88,7 +88,4 @@ public:
 
 bool isUri(const string & s);
 
-/* Decode data according to the Content-Encoding header. */
-ref<std::string> decodeContent(const std::string & encoding, ref<std::string> data);
-
 }