PG:: reset interval_flush and last_peering_reset in GetInfo, not 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.

We don't really want to reset the last_peering_interval in the Reset
constructor anyway since we always repost the AdvMap event causing
start_peering_interval() to fire in the Reset AdvMap handler.  That always
resets last_peering_reset and interval_flush anyway.  Instead, lets reset
last_peering_reset and interval_flush in GetInfo before transitioning to
Reset.

Fixes: #9821
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>