PG: set !flushed in Reset()

Otherwise, we might serve a pull before we start_flush in the
ReplicaActive constructor.

Fixes: #5799
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 9e7d6d547e0e8a6db6ba611882afa9bf74ea0195)