get rid of internal_config feature

[milyin]

Describe the release item

The feature internal_config can be removed, it's functionality can be put under internal feature.

Though to achieve this goal one of these things should be done:

• either the internal should not suppose unstable. E.g. test for liveliness which actively uses this write access to config should use internal feature, but not unstable. This is nesessary for run tests in stable mode #1654
• or the tests should not use this direct access to config, using set methods instead