It runs in the background and consists of two parts -- async() lambda and following .then()-s. This PR move the background running code into its own method and coroutinizes it in parts. With #19954 merged it finally looks really nice.
Closesscylladb/scylladb#20058
* github.com:scylladb/scylladb:
view_builder: Restore indentation after previous patches
view_builder: Coroutinize inner start_in_background() calls
view_builder: Coroutinize outer start_in_background() calls
view_builder: Add helper method for background start