[PATCH] Add `truncate` option to subscription commands

-hackers,

Enclosed find a patch to add a “truncate” option to subscription commands.

When adding new tables to a subscription (either via `CREATE SUBSCRIPTION` or `REFRESH PUBLICATION`), tables on the
targetwhich are being newly subscribed will be truncated before the data copy step.  This saves explicit coordination
ofa manual `TRUNCATE` on the target tables and allows the results of the initial data sync to be the same as on the
publisherat the time of sync. 

To preserve compatibility with existing behavior, the default value for this parameter is `false`.

Best,

David



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David Christensen
Senior Software and Database Engineer
End Point Corporation
david@endpoint.com
785-727-1171