Scan refactor

[0bsearch]

Since scan search-type is deprecated since 2.1, there's simpler implementation for scan logic.

Also adds tests for scan-related stuff

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[fxdgear]

jenkins test this please

[0bsearch]

Side note on this: for next major release I propose to change this one step further — let's make error-handling run before yielding results. This will not affect case with raise_on_error=False, but will introduce fast exit route for raise_on_error=True. IMO, raising exception as soon as we see problem in response makes more sense that yielding batch of results first.

[fxdgear]

i've yet to do the 7.x release, so if you want to do that now feel free.