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
blob: removed paranoid check from _eblob_read_ll
blob: improved perfomance of eblob_read_data
mobject: removed unused variable
tests: stress: removed idempotent operation