Store persistent reservation keys as uint64_t instead of uint8_t[8].

This allows to simplify the code and save 512KB of RAM per LUN (8%)
by removing no longer needed "registered" keys flags.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272893 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f