use PHP's pack function to handle write_long

Using the library to process thousands of messages and profiling the execution time, I realize that AMQPWriter::chrbytesplit was a huge pain point.

In the case of write_long, looks that we can simply use PHP's pack function, which is straightforward and faster.

But maybe AMQPWriter::chrbytesplit was here for a good reason. If so please tell why :-)