crimson/osd: merge pg_temp_wanted before sending them

crimson/osd: merge pg_temp_wanted before sending them

there is chance that new pg_temp_wanted is added when we are sending
them in ShardServices::send_pg_temp(), but the pg temp are already
collected into the messages before seastar::parallel_for_each(). so,
we cannot move all elements from pg_temp_wanted to pg_temp_pending after
sending the messages, that'd also move the unsent pg temps to
pg_temp_pending as well. so we have to do this before do the async call,

this issue was root caused by Xuehan Xu <xxhdx1985126@gmail.com>.

Signed-off-by: Kefu Chai <kchai@redhat.com>