tablet server uses this flag enable_distsql to enable cluster optimization

. It is on with release configurations: release/conf/tablet.flags.template, but no in src/flags.cc. Which as a results, won't enabled in mini_cluster tests, e.g sql_sdk_test.

There is a PR previously #3295, but turns out breaks existing tests. It means there exists cases that is OK with request mode, but not request(cluster) mode.

#3531 is a example, deployment tests not fail for those tests, simply because enable_distsql not turned on.