mon: Elector: bootstrap on new monmap from elector

mon: Elector: bootstrap on new monmap from elector

Whenever we update the monmap we should bootstrap, in order to reset the
monitor's on-going activities and re-probe.

Not doing so contributed to bug #3252, during which we entered an infinite
election cycle. This may only happen though when we rely on 'mon initial
peers'. Specifying a monmap during the monitor's mkfs should not trigger
this bug.

Fixes: #3252
Backport: argonaut

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