Missing changes in swap_head_or_index_files()

[za-arthur]

This is follow up ticket. See the another ticket #152 and pull requests #157, #377.

It seems to me that besides swapping relfrozenxid, relfrozenxid and relallvisible there are other changes which are missing in swap_head_or_index_files() comparing to Postgres swap_relation_files().

We don't need to migrate some code:

• AFAIK pg_repack doesn't support mapped tables.
• pg_repack doesn't repack system tables.

At this point I'm not sure if we need to migrate the following changes:

• Call RelationClearMissing(). It seems it isn't harm to not call it. What it does is that it sets atthasmissing to false, which means that there are no values in a tables which miss default value of an attribute.
• Support swap_toast_by_content. I'm not sure if it is necessary to support swapping toast tables by content. See the code where Postgres decides when to swap toast tables by content.

And it might be good to do the following changes:

• Swap relam. pg_repack currently might not support changes access methods, but it might be better to sync this change. In that case it might be necessary to update AM dependencies.
• Same for relpersistence..

I think that the following changes are important and needs to be migrated:

• Call RelationAssumeNewRelfilelocator() (or RelationAssumeNewRelfilenode() on older versions). According to the comment of the function it is necessary to call it if relfilenode is changed.
• Call RelationCloseSmgrByOid(). I'm not sure about this part yet.