elector: trigger a mon reset whenever we bump the epoch

We need to call reset during every election cycle; luckily we
can call it more than once. bump_epoch is (by definition!) only called
once per cycle, and it's called at the beginning, so we put it there.

Fixes #4858.

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>