rgw: pass boost intrusive_ptr to DataLogBackends flat_map

rgw: pass boost intrusive_ptr to DataLogBackends flat_map
When emplacing a new backend, use an explicitly constructed
intrusive_ptr so that it gets destroyed if the emplace fails.

Though the destructor is already called, this behaviour cannot be relied
upon. std::map does not exhibit the same behaviour, and would have
leaked memory had it been used instead.

Signed-off-by: Vedansh Bhartia <vedanshbhartia@gmail.com>