Removed index_offset in eblob_base_ctl, replaced its use by index_size.

In most cases `index_offset` was equal to `index_size` and defined where new record should be written. Only case when `index_offset` wasn't equal to `index_size` was `index_offset` of closed blob. It was equal to 0. This situation had made confusion, so it was decided to remove `index_offset`.
Also there was satisfied TODO comment and used `__eblob_read_ll()` instead of `pread()`.