Features for more customized behavior

Were added:
* Iterations over Secondary Indexes - name of the index to iterate on
* Transaction support - ability to process tuples from each batch in a single transaction
* Start key - start iterating from the tuple with this index value
* Iterator type - type of the iterator to use, as string or box.index constant
* Process while function - function to call before checking each tuple, if it returns false, the task will stop until next full scan
* And the ability to write custom iterator behavior for the user - `iterate_with` function which returns an iterator object which provides tuples to check

Additional changes:
Rewritten space iteration for tree index from select to pairs since the iterator is now stable
Added some comments and removed duplicate code(expirationd_kill_task)
Described new parameters in the code

Closes: #50

Co-authored-by: Nick Volynkin <nick.volynkin@gmail.com>

Author: ArtDu