Refactoring: extract logic of rake tasks into its own classes

This PR extracts the logic of rake tasks into smaller separate classes that can be tested separately. These changes also prepare a better reuse of the functionality, e.g. CLI client.

* the logic to dump index, upload seed file were extracted to its own classes
* refactored chunked sender, split into algorithm and a sink that sends data to ES index, better separation of concerns
* added rake tasks to delete index / template
* included RSpec and added specs