Fix TAM segfault on DELETE using segmentby column

If a whole segment is deleted in a Hypercore TAM table, a segmentation
fault can be triggered that results from reading freed data.

After calling `table_open()` and locking the table in
`LockRelationOid()` all invalidation messages will be processed. This
can invalidate the `rd_amcache` field for the relation, but also for
other relations.

As a result, this caused clobbered data to be used when finding the
count column in the compressed relation since the attribute number was
0x7f7f when first fetching HypercoreInfo using
`RelationGetHypercoreInfo()`, calling `table_open()`, and then trying
to use the data in the structure again.

This commit fixes this by not re-using data returned by
`RelationGetHypercoreInfo()` after calling `table_open()`. Since one of
the more common cases for this is opening the compressed table beloning
to a chunk, a new function were introduced for handling this case.

It also moves calls to `RelationGetHypercoreInfo()` inside functions
that need the data to avoid re-using the data after calls that can
invalidate the cache.

Author: mkindahl