[vtgate] Fails to route any query when keyspace filtering is enabled

[setassociative]

Overview of the Issue

When we started working on online DDL support as a first-party action we introduced a dependency in the vtgate to be able to modify the topology. This conflicts with limitations enforced by a KeyspaceFilteringServer which has access to only a subset of the topo.

As a protective measure when attempting to gain access to the Write-enabled topo.Server a filtering server throws an error. Since this is now happening as part of the vcursor setup to support schema changes all vtgate routing was failing if -keyspaces_to_watch was passed to the vtgate.

Reproduction Steps

The summary:

1. start vtgate with -keyspaces_to_watch commerce
2. attempt to perform a query

I ended up validating my change by modifying examples/local/scripts/vttablet-up.sh to pass the keyspace filtering flag and then using the sample examples/local/101_initial_cluster.sh to bootstrap everything.

When making a query you get a result back like:

[~/vitess/examples/local] (vtgate_keyspaces_to_watch)
sa@docker $ mysql -S /tmp/mysql.sock
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.9-vitess-11.0.0-SNAPSHOT

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MySQL [(none)]> select 1 from dual;
ERROR 1105 (HY000): cannot access underlying topology server when keyspace filtering is enabled

Binary version

This is present in all releases >= 8.0, I don't recall off hand which PR actually introduced it.