Drop elem cache is broken on init() call

[tadatuta]

Methods like append and prepend won't drop elem cache as there's a few issues:

• dropElemCacheQueue is set to undefined for custom context here https://github.com/bem/bem-core/blob/v4/common.blocks/i-bem-dom/i-bem-dom.js#L970 but it's expected to be mutated at https://github.com/bem/bem-core/blob/v4/common.blocks/i-bem-dom/i-bem-dom.js#L974

• Only DOM nodes with i-bem class are taken into account here https://github.com/bem/bem-core/blob/v4/common.blocks/i-bem-dom/i-bem-dom.js#L973

• Only entities mentioned in data-bem affect cache (see https://github.com/bem/bem-core/blob/v4/common.blocks/i-bem-dom/i-bem-dom.js#L101).