Zero partition helper

What does this MR do and why?

Describe in detail what your merge request does and why.

Helper for list-partitioning an existing table and attaching it as the first partition of a new table.

This helper performs the following actions:

• verifies that the table is partitionable (a check constraint already supports the partitioning, and all unique are compatible
• creates a new parent table to attach the current table to
• Adds foreign keys to the new parent table, taking care to lock only one other table per transaction
• Attaches the table as a first partition of the parent
• Re-owns sequences so that they belong to the parent
• Drops the check constraint that supported the partitioning, as it is now redundant.

The entire process is designed to take as few locks as possible per-transaction, and to be rerunnable if a failure occurs at any point.

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