api: simplified prepare/plain/commit

Do not accept write_control only extract few fields from it.
This greatly simplifies API but this is non-backward compatible change.

Also in future releases we can add special *_return conterparts to
speedup common work flows.

Also plain write does not commit changes on each write.
api: unified API

Provide both:
    1) Simple eblob_writev() for end users and
    2) Extended eblob_writev_return() for elliptics
config: removed bsize and cache_size config options

Also added different types of padding to eblob_config structure. From
now on it'll be easier to add new fields to it.
python: whitespace fixes
misc: comment fixes
stat: moved statistics related decls into stat.h
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