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.
Each test_iteration_* method have an description of what will be iterated and what will be checked. I will update these comments with your corrections. eblob_stress includes 3 iteration cases:
* iterating ranges that are supposed to contain no existent keys
* iterating ranges that covers some part of existent keys
* iterating one range that includes all existent keys

After each iteration it is checked that all required key are iterated or that no key has been iterated if it required by test.