ipc: put lz4 decompression buffers back into sync.Pool

The lz4 decompressor was not calling Reset on the underlying writer in its
Close method. This could cause buffers not to be released back to the pool and
defeating the purpose of the sync.Pool in the lz4 package.

Additionally, a call to Close was missing in readDictionary.

go/arrow/ipc/compression.go

@@ -104,7 +104,9 @@ type lz4Decompressor struct {
 	*lz4.Reader
 }
 
-func (z *lz4Decompressor) Close() {}
+func (z *lz4Decompressor) Close() {
+	z.Reader.Reset(nil)
+}
 
 func getDecompressor(codec flatbuf.CompressionType) decompressor {
 	switch codec {

go/arrow/ipc/file_reader.go

@@ -730,6 +730,7 @@ func readDictionary(memo *dictutils.Memo, meta *memory.Buffer, body ReadAtSeeker
 	bodyCompress := data.Compression(nil)
 	if bodyCompress != nil {
 		codec = getDecompressor(bodyCompress.Codec())
+		defer codec.Close()
 	}
 
 	id := md.Id()