Added iteration tests to eblob stress.

At the end of eblob stress it will runs 3 iterations:
##### Out of range iteration
Runs iteration on ranges: `[00..0, minimal key)` and `(maximal key, FF..F]`. These ranges do not include any of existent key. Checks that iteration does not find any key.

##### Part of range iteration
Creates 2 ranges: `[00..0, 40..0]` and `[C0..0, F0..0]`, that should include some part of existent key. Find such keys and checks that iteration finds all these key.

##### Full range iteration
Runs iteration on one range: `[00..0, FF..F]` and checks that all existent keys have been found by iteration.

These tests found that iteration of unsorted indexes returns keys from out of ranges - I've added checking ranges even if index is unsorted and used `bsearch` for checking if a key is from ranges.
I've used bsearch instead of this linear search and put comments on comparison function eblob_key_range_compare. I will  put additional comments on bsearch usage.