This way, you don't need to always fetch it from the system catalogs,
which makes things simpler, and is marginally faster too.

To make all the fields in pg_appendonly accessible by direct access to
the Form_pg_appendonly struct, change 'compresstype' field from text to
name. "No compression" is now represented by an empty string, rather than
NULL. I hope there are no applications out there that will get confused
by this.

The GetAppendOnlyEntry() function used to take a Snapshot as argument,
but that seems unnecessary. The data in pg_appendonly doesn't change for
a table after it's been created. Except when it's ALTERed, or rewritten
by TRUNCATE or CLUSTER, but those operations invalidate the relcache, and
we're never interested in the old version.

There's not much need for the AppendOnlyEntry struct and the
GetAppendOnlyEntry() function anymore; you can just as easily just access
the Form_pg_appendonly struct directly. I'll remove that as a separate
commit, though, to keep this one more readable.