sql/multitenancy: unexpected concurrency for flow error when creating stats

[asubiotto]

Seen in enums logic test when run with 3node-tenant config:

CREATE STATISTICS s FROM greeting_stats;
    -- FAIL
    TestLogic/3node-tenant/enums: logic.go:2251:

        testdata/logic_test/enums:368:
        expected success, but found
        (XX000) internal error: unexpected concurrency for a flow that was forced to be planned locally
        distsql_running.go:403: in Run()
        DETAIL: stack trace:
        github.com/cockroachdb/cockroach/pkg/sql/distsql_running.go:403: Run()
        github.com/cockroachdb/cockroach/pkg/sql/distsql_plan_stats.go:257: planAndRunCreateStats()
        github.com/cockroachdb/cockroach/pkg/sql/create_stats.go:427: func2()
        github.com/cockroachdb/cockroach/pkg/kv/db.go:707: func1()
        github.com/cockroachdb/cockroach/pkg/kv/txn.go:800: exec()
        github.com/cockroachdb/cockroach/pkg/kv/db.go:706: Txn()
        github.com/cockroachdb/cockroach/pkg/sql/create_stats.go:414: Resume()
        github.com/cockroachdb/cockroach/pkg/jobs/registry.go:821: stepThroughStateMachine()
        github.com/cockroachdb/cockroach/pkg/jobs/registry.go:952: func1()
        github.com/cockroachdb/cockroach/pkg/util/stop/stopper.go:323: func1()
        runtime/asm_amd64.s:1373: goexit()

We probably don't check planCtx.isLocal (which is forced in a tenant) somewhere we should be.

[asubiotto]

This happens because we force local planning of all flows as a tenant. Stats collection processors do not implement RowSource (they only implement Run) and so cannot be "fused" together. This inability to fuse the processors is what causes this error.

The error's goal is to protect against concurrent usage of the RootTxn which wouldn't happen in this case. Here two solutions:

1. Make this check more lenient. We could whitelist processors by having them implement SafeForConcurrentLocalExecution. This'll be the easiest cc @andreimatei since you introduced this check.
2. Implement these processors as RowSources so that they can be fused. Was there any reason this wasn't done before? cc @rytaft