use libbrotli directly when available

* Look for both 'brotli' and 'bro' as external command,
  since upstream has renamed it in newer versions.
  If neither are found, current runtime behavior
  is preserved: try to find 'bro' on PATH.
* Limit amount handed to BrotliEncoderCompressStream
  to ensure interrupts are processed in a timely manner.
  Testing shows negligible performance impact.
  (Other compression sinks don't seem to require this)

tests/brotli.sh

@@ -0,0 +1,28 @@
+source common.sh
+
+
+# Only test if we found brotli libraries
+# (CLI tool is likely unavailable if libraries are missing)
+if [ -n "$HAVE_BROTLI" ]; then
+
+clearStore
+clearCache
+
+cacheURI="file://$cacheDir?compression=br"
+
+outPath=$(nix-build dependencies.nix --no-out-link)
+
+nix copy --to $cacheURI $outPath
+
+HASH=$(nix hash-path $outPath)
+
+clearStore
+clearCacheCache
+
+nix copy --from $cacheURI $outPath --no-check-sigs
+
+HASH2=$(nix hash-path $outPath)
+
+[[ $HASH = $HASH2 ]]
+
+fi # HAVE_BROTLI