auth/cephx/CephxSessionHandler: implement CEPHX_V2 calculation mode

auth/cephx/CephxSessionHandler: implement CEPHX_V2 calculation mode

Derive the signature from the entire buffer (both cipher blocks).

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 8f396cf35a3826044b089141667a196454c0a587)

# Conflicts:
#	src/auth/cephx/CephxSessionHandler.cc

- master and mimic use no_bl crypto interface, which doesn't exist here,
so fall back to using bufferlist-based calls into encrypt().