osd/Watch: use vector<> instead of list<>

osd/Watch: use vector<> instead of list<>

there is no need to use a list<> here. and this change does not
change the encoding schema here. also, as a fact, the user of the
watch-notify feature is decoding the bufferlist like:

 std::map<std::pair<uint64_t,uint64_t>, bufferlist> reply_map;
 std::set<std::pair<uint64_t,uint64_t> > missed_map;
 decode(reply_map, reply_p);
 decode(missed_map, reply_p);

so, in this change, change list<> to vector<> for both smaller memory
footprint and for better readability.

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