Unable to drop keyspace or table if there are active secondary indexes

[penberg]

Installation details
Scylla version (or git commit hash): 2.1-rc3

Apache Cassandra lets users run DROP KEYSPACE and DROP TABLE even if there are secondary indexes but Scylla trips over materialized views validation (that SI uses under the hood) and refuses that.

[psarna]

There are two separate issues here, I think they should be split.

1. Dropping a table with secondary indexes - I'll send a patch series for that soon. The issue is that dropping a table should be allowed even if it has secondary indexes.

2. Dropping a keyspace with any materialized views present, not just secondary indexes.
   Here's a reproduction:

CREATE KEYSPACE demodb WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 1 };
use demodb;
CREATE TABLE tbl (a int, b int, c float, PRIMARY KEY (a));
CREATE MATERIALIZED VIEW tbl_view AS SELECT c FROM tbl WHERE c IS NOT NULL PRIMARY KEY (c, a);

cqlsh:demodb> drop keyspace demodb;
ServerError: Can't find a column family with UUID 75728b10-5da8-11e8-9297-000000000000
cqlsh:demodb> drop keyspace demodb;

If any materialized views are present, first attempt to drop this keyspace will fail with ServerError: Can't find a column family with UUID. Still, all tables and views are dropped from it and the second attempt succeeds without any problems. I think it should be moved to a separate issue, it's not directly related to secondary indexes.