TS-1851: Remove HostDBInfo constructor so it becomes a POD

Somewhere down the line HostDBInfo grew enough constructor complexity
that it became a non-POD type. The allocation treats it like a POD
type, so we should make sure that the compiler agrees.

Rename HostDBRoundRobin::n to HostDBRoundRobin::rrcount. Remove
unused HostDBInfo::set_from