connectedCallback called multiple times for DCE child

[sashafirsov]

http-request and perhaps other DCE components connected to DOM twice:

• during custo-element connectedCallback on template content re-attachment
• after template render

As the initial content has XSLT markup like <http-requst url="{url}", the attributes and content is not valid.

The template extraction should be not following the DOM re-injection, rather it has to be used as the base for XSLT and later the rendered payload replacing the <template>.