Add an ability to create indexes in AVAILABLE state

Currently we have a special flag for PK indexes that indicate that they are created alongside its table and are not required to be built later by a background task. Instead, such indexes are filled with data as soon as the data is inserted into the parent table.

I propose to generalize this mechanism, so that we can create arbitrary indexes at the same time as their parent tables. This may prove useful in the future, if we allow to create indexes inside the "CREATE TABLE" statement, for example.