Refactor inbatch_parallel

[SergeyTsimfer]

As the inbatch_parallel now not supposed to be used on its own, we can refactor it with following goals in mind:

• remove _use_self args
• remove init/post functions: the container with init should be passed directly from Batch.apply_parallel, and the results should be post-processed in the Batch.apply_parallel as well
• make inbatch_parallel a class: that would allow for easier introspection and parameter changes on the fly, for example, target to any other.

[SergeyTsimfer]

use inspect to check whether src/dst should be passed to the method

[roman-kh]

Avoid inspect if possible. Use inbatch_parallel arguments instead.