Elector: keep a list of classic mons instead of each mon's commands

We aren't actually using the sets, so don't bother keeping them.

Signed-off-by: Greg Farnum <greg@inktank.com>
Monitor: encode and expose mon command sets

Signed-off-by: Greg Farnum <greg@inktank.com>
Elector: use monitor's encoded command sets instead of our own

Signed-off-by: Greg Farnum <greg@inktank.com>