Increase MULTIPART_CHUNK_SIZE

Increasing the chunk size avoids an issue where too many parts are uploaded, triggering an error when calling the S3 CompleteMultipartUpload API command.  The current limit on the number of parts per upload is 10,000 according to the S3 documentation (https://docs.aws.amazon.com/AmazonS3/latest/dev/qfacts.html)

src/s3.c

@@ -2057,7 +2057,7 @@ static int putObjectDataCallback(int bufferSize, char *buffer,
     return ret;
 }
 
-#define MULTIPART_CHUNK_SIZE (15 << 20) // multipart is 15M
+#define MULTIPART_CHUNK_SIZE (768 << 20) // multipart is 768M
 
 typedef struct MultipartPartData {
     put_object_callback_data put_object_data;