Initial transition to sdb: anal.xrefs

- Cascade core_project_save > anal_project_save > anal_xrefs_save
- Database is stored into ~/.radare2/rdb/$prj.d/xrefs
- Build libsdb properly to get access to all the api
- Use sdb from r_anal_xrefs() - needs cleanup
- Update sdb to the latest version from git
- Fix some warnings