Wip rados whereis

This includes/is based on wip-table-formatter https://github.com/ceph/ceph/pull/2728#

It adds a RadosWhereis class to get access to the location information of objects and the on/off state of the hosting osd. It supports hostname reverse dns using the boost library.

Usage:
```
std::vector<librados::whereis_t> locations;
int retc;
if ((retc = librados::Rados::whereis(io_ctx, oid, locations))) {
   cerr << "failed to locate " << pool_name << "/" << oid << ": " << cpp_strerror(ret) << std::endl;
}
```
Since I don't know which output to expect I didn't add a unit test for this.