db: schema_tables: futurize and coroutinize merge_tables_and_views()
db: schema_tables: futurize and coroutinize merge_tables_and_views()

Right now, merge_tables_and_views() expects to be called in a thread.
Remove that requirement by converting it into a coroutine and returning
a future.

De-threading helps reduce errors where something expects to be called
in a thread, but isn't.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla