crimson/osd/osd_operations/snaptrim_event: don't process snaptrim events

crimson/osd/osd_operations/snaptrim_event: don't process snaptrim events
of the same PG concurrently

Concurrently triming snaps of the same PG may lead to the following
problem:

1. fiber 1 get objects to trim for PG 1.x and snapid 1.d, which involves
   object OBJ;
2. fiber 2 get objects to trim for PG 1.x and snapid 1.c, which also
   involes object OBJ;
3. fiber 1 removes snap 1.c since it has been removed in the osdmap
4. fiber 2 try to get obc for object OBJ and find that OBJ doesn't have
   snap for id 1.c

Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>