stats: full rewrite

Now we have 3 types of stats:
    1) Global per backend stats, e.g. number of hased entries
    2) Local per base stats, e.g. number of records in base
    3) Summary stats across all bases of backend e.g. total number of
        records in backend.

Also added some decent APIs for statistics manipulation and extraction:
    eblob_stat_add, eblob_stat_set, eblob_stat_get
And some number of common shortcuts:
    eblob_stat_inc, eblob_stat_dec
defrag: do not run iterator in eblob_want_defrag()

This will also speed up data-sort, but from now on we should be very
carefull to have actual stats for bases to not accidientally remove
non-empty base.
cleanups: removed unused fields from bctl