JIRA: https://jira.spring.io/browse/INT-4262
Since `IF EXISTS` doesn't exist for `DROP INDEX` command and even more
this command isn't necessary when we fully drop the table, we don't need it
as a separate command.
`DROP TABLE IF EXISTS` is fully sufficient for schema recreation requirements
* Add `NODROPINDEX` variable for VPP engine to let it do not add `DROP INDEX` command
* Fix `channel/schema-mysql.sql` for proper INDEX names
ecc6824a8c