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/s3-binary-cache-store.cc

@@ -153,10 +153,8 @@ S3Helper::DownloadResult S3Helper::getObject(
         auto result = checkAws(fmt("AWS error fetching '%s'", key),
             client->GetObject(request));
 
-        res.data = decodeContent(
-            result.GetContentEncoding(),
-            make_ref<std::string>(
-                dynamic_cast<std::stringstream &>(result.GetBody()).str()));
+        res.data = decompress(result.GetContentEncoding(),
+            dynamic_cast<std::stringstream &>(result.GetBody()).str());
 
     } catch (S3Error & e) {
         if (e.err != Aws::S3::S3Errors::NO_SUCH_KEY) throw;