Fixes#5582
... but only populate log on shard 0.
Migration manager callbacks are slightly assymetric. Notifications
for pre-create/update mutations are sent only on initiating shard
(neccesary, because we consider the mutations mutable).
But "created" callbacks are sent on all shards (immutable).
We must subscribe on all shards, but still do population of cdc table
only once, otherwise we can either miss table creat or populate
more than once.
v2:
- Add test case
Message-Id: <20200113140524.14890-1-calle@scylladb.com>
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