Use sun.misc.Unsafe to access a direct buffer if possible

This pull request introduces a new `PooledByteBuf` implementation that uses `sun.misc.Unsafe` wherever possible when using a direct buffer.  In my experiment, it seems to perform slightly better than the old implementation.  I'd love to know what your test result looks like.  I used the `HttpSnoopServer` and httperf to test the throughput difference.