Cache hobject filestore_key to avoid massive _reverse_nibbles calls

In a simple 4k object WriteFull operation process, the hobject::_reverse_nibbles is called 900 times. In my environment which provides 500 IOPS, the hobject::_reverse_nibbles is called 270K times per second , which cost lots of CPU.

This patch will cache the _reverse_nibbles result in a field of hobject struct. And this will reduce the call requests of _reverse_nibbles to about 30 per IO.
may as well use set_hash() here