Features for more customized behavior

[ArtDu]

• 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

Test coverage added.

RFC: #50

[olegrok]

I give up. Probably we should start to file issues for some comments. Otherwise there is a risk that this patch never will be merged.

[ligurio]

• Limit commit message width in commits "Iterate_with and process_while" and "Start key and iterator type"
• We decided to move some requested on review changes to the backlog. Please make sure all appropriated tickets submitted.
  • JSON-path index error Json-path index as primary key doesn't work #59
  • Unclear options Unclear options #60
  • Convert TAP-style tests to luatest-style test Rewrite tests from tap to luatest #61
• We need to enable new tests as soon as you added them. So put luatest related changes to commit
  where first test is appeared (it is "Iterate over any index").
• Changes are not atomic now. I see commits with new functionality without any documentation in a README.
  If we don't need to document these changes then please explain why.
• Why do you mix double quotes and single quotes in a whole patchset?
• Latest commit should contain a "Closes RFC: Adding Iterations over Secondary Indexes  #50", otherwise it is not clear finished task or not.

[ArtDu]

• Limit commit message width in commits "Iterate_with and process_while" and "Start key and iterator type"

• We decided to move some requested on review changes to the backlog. Please make sure all appropriated tickets submitted.

  • Unclear options Unclear options #60
  • Convert TAP-style tests to luatest-style test Rewrite tests from tap to luatest #61

• We need to enable new tests as soon as you added them. So put luatest related changes to commit
  where first test is appeared (it is "Iterate over any index").

• Changes are not atomic now. I see commits with new functionality without any documentation in a README.
  If we don't need to document these changes then please explain why.

• Why do you mix double quotes and single quotes in a whole patchset?

• Latest commit should contain a "Closes RFC: Adding Iterations over Secondary Indexes  #50", otherwise it is not clear finished task or not.

• Reduced the body size of all commits to 72 characters.
• Luatest is added to ci with the first appearance of tests ("Iterate over any index" commit).
• Changes are atomic now. Each commit on changing the functionality contains the addition of functionality, its description in the readme and testing it in the luatest.
• Mix double quotes and single quotes were not in the code. The outer ones are double quotes, the same style has been added to the README for greater consistency.
• Added closes: RFC: Adding Iterations over Secondary Indexes  #50 in last commit body message.