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.
key **is** NOT supposed

Please update other error messages too. And write a small description of what this function does at the beginning in doxygen format, like 'we have list of items which are either present or removed in the blob, iterator should find those keys and compare whether remove/exist status matches' or something like that