mon: upgrade auth database on leader

If we are the leader, and the auth database has not yet been upgraded,
do so.  The upgrade consists of translating old-style (pre-v0.64) caps
to new-style caps (e.g., 'allow profile bootstrap-osd').  This happens
once and the conversion takes the form of a normal paxos transaction.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>