release-21.1: changefeedccl: fail changefeeds when tables go offline

[stevendanna]

Backport 1/1 commits from #64323.

/cc @cockroachdb/release

---

In 20.2.4, a changefeed would fail if IMPORT INTO was run against one
of its target tables. The failure would look like:

I210428 10:45:57.982012 2015 jobs/registry.go:1131 ⋮ [n1] CHANGEFEED job 653840730248282113: stepping through state failed with error: ‹relation› ‹"test"› is offline: ‹importing›
(1)
Wraps: (2) attached stack trace
  -- stack trace:
  | github.com/cockroachdb/cockroach/pkg/sql/catalog.FilterDescriptorState
  |     /Users/ssd/go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descriptor.go:387
  | github.com/cockroachdb/cockroach/pkg/sql/catalog/lease.storage.acquire.func1
  |     /Users/ssd/go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/lease/lease.go:219
  | github.com/cockroachdb/cockroach/pkg/kv.(*DB).Txn.func1
  |     /Users/ssd/go/src/github.com/cockroachdb/cockroach/pkg/kv/db.go:758
  | github.com/cockroachdb/cockroach/pkg/kv.(*Txn).exec
  |     /Users/ssd/go/src/github.com/cockroachdb/cockroach/pkg/kv/txn.go:808
  | github.com/cockroachdb/cockroach/pkg/kv.(*DB).Txn
  |     /Users/ssd/go/src/github.com/cockroachdb/cockroach/pkg/kv/db.go:757
  | github.com/cockroachdb/cockroach/pkg/sql/catalog/lease.storage.acquire
  |     /Users/ssd/go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/lease/lease.go:193
  | github.com/cockroachdb/cockroach/pkg/sql/catalog/lease.acquireNodeLease.func1
  |     /Users/ssd/go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/lease/lease.go:859
  | github.com/cockroachdb/cockroach/pkg/util/syncutil/singleflight.(*Group).doCall
  |     /Users/ssd/go/src/github.com/cockroachdb/cockroach/pkg/util/syncutil/singleflight/singleflight.go:128
  | runtime.goexit
  |     /usr/local/Cellar/go/1.16.3/libexec/src/runtime/asm_amd64.s:1371
Wraps: (3) ‹relation› ‹"test"› is offline: ‹importing›
Error
types: (1) *catalog.inactiveDescriptorError (2) *withstack.withStack (3) *errutil.leafError

We want changefeed to fail when IMPORT INTO is run because changes via
the AddSSTable mechanism is not currently reflected in the changefeed,
meaning we would fail to emit imported data.

The previous path that raised this failure depended on:

1. The descriptor being offline at the point we attempted to acquire a
   lease on it:

   cockroach/pkg/ccl/changefeedccl/schemafeed/schema_feed.go

   Line 514 in d196291

   if err := tf.leaseMgr.AcquireFreshestFromStore(ctx, desc.ID); err != nil {

2. The lease acquisition filtering out offline descriptors with an
   error:

   cockroach/pkg/sql/catalog/descriptor.go

   Line 209 in eda2309

   err = errors.Errorf("%s %q is offline: %s", desc.TypeName(), desc.GetName(), desc.GetOfflineReason())

3. The failure from the lease acquisition in the schemafeed being
   treated as a fatal error.

I believe our behaviour here has changed a few times on both the 20.2
branch and master because of changes in each of these 3 behaviours.

In this change, rather than relying on the lease acquisition, we
specifically check for offline tables in our ValidateTable
function. This function is called for every descriptor version we get
from the ExportRequest on the Descriptor table.

Currently, I believe that checking for the offline descriptors is
correct since it appears that only restore and import put tables into
an offline state.

Release note (enterprise change): CHANGEFEEDs more reliably fail when
IMPORT INTO is run against a targeted table.

Fixes #64276

See also #62585, #43784

[cockroach-teamcity]

This change is