Allow table AM's to cache stuff in relcache

Index AM's can cache stuff in RelationData->rd_amcache. In the zedstore
table AM we've been hacking on, I'd like to also use rd_amcache to cache
some information, but that's not currently possible, because rd_amcache
can only be used by index AMs, not table AMs.

Attached patch allows rd_amcache to also be used by table AMs.

While working on this, I noticed that the memory management of relcache
entries is quite complicated. Most stuff that's part of a relcache entry
is allocated in CacheMemoryContext. But some fields have a dedicated
memory context to hold them, like rd_rulescxt for rules and rd_pdcxt for
partition information. And indexes have rd_indexcxt to hold all kinds of
index support info.

In the patch, I documented that rd_amcache must be allocated in
CacheMemoryContext, or in rd_indexcxt if it's an index. It works, but
it's a bit weird. It would nice to have one memory context in every
relcache entry, to hold all the stuff related to it, including
rd_amcache. In other words, it would be nice if we had "rd_indexcxt" for
tables, too, not just indexes. That would allow tracking memory usage
more accurately, if you're debugging an out of memory situation for example.

However, the special contexts like rd_rulescxt and rd_pdcxt would still
be needed, because of the way RelationClearRelation preserves them, when
rebuilding the relcache entry for an open relation. So I'm not sure how
much it would really simplify things. Also, there's some overhead for
having extra memory contexts, and some people already complain that the
relcache uses too much memory.

Alternatively, we could document that rd_amcache should always be
allocated in CacheMemoryContext, even for indexes. That would make the
rule for pg_amcache straightforward. There's no particular reason why
rd_amcache has to be allocated in rd_indexcxt, except for how it's
accounted for in memory context dumps.

- Heikki