Do no longer scan chunks during query planning to determine the schema
(except for the lifetime jobs where we have a good reason to do so).
Instead pass the schema down to from whoever is triggering the query.
For real SQL queries, we then just use the the table-wide schemas
introduced in #1913.
Apart from avoiding schema merges we now also don't crash any longer
when no chunks are left in the table (aka columns are present but all
rows are gone).
Fixes#1768.
Fixes#1884.
Marco Neumann