PG:: reset_interval_flush and in set_last_peering_reset

If we have a change in the prior set, but not in the up/acting set, we go back
through Reset in order to reset peering state.  Previously, we would reset
last_peering_reset in the Reset constructor.  This did not, however, reset the
flush_interval, which caused the eventual flush event to be ignored and the
peering messages to not be sent.

Instead, we will always reset_interval_flush if we are actually changing the
last_peering_reset value.

Fixes: #9821
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>