Add keyspace option to DROP INDEX statement

Actually drop index statement syntax is:

DROP INDEX IF EXISTS index_name

However, keyspace can be specified in drop table statement:

DROP TABLE IF EXISTS keyspace_name.table_name

And it's also used in index creation statement:

CREATE CUSTOM INDEX IF NOT EXISTS index_name
ON keyspace_name.table_name ( column_name )
USING (class_name) (WITH OPTIONS = map)

I think it could be useful add the optional keyspace name to the drop index statement:

DROP INDEX IF EXISTS keyspace_name.index_name

Thus it would be possible to delete an index without using the USE statement. I attach a patch for doing this.