- added ->md_config() and ->dt_config() method for getting/setting named configuratio...

- added ->md_config() and ->dt_config() method for getting/setting named configuration data from/to back store device. For instance, MDT needs to get saved meta-sequence info from lower layers (osd);

- added mdt_metaseq_init() method called from mdt_init0();
- added basic and primitive stuff about meta-sequence management in mdt;
- added some comments, fixes in mdt_init0() about correct handling error cases (memort freeing, etc.);
- fixed memory leak in mdt_fini(). There d->ld_site was not freed;