Index-only-scans, indexam API changes

At the moment, amgettuple only returns pointers to heap tuples. There is
no way to return data from the index tuples. That needs to be changed to
support index-only scans.

I propose that we split index_getnext into two steps: fetching the next
match from the index (index_next()), and fetching the corresponding heap
tuple (index_fetch()). Patch attached. There is still a shorthand
index_getnext() that is compatible with the old function, but it now
just calls index_next()+index_fetch().

The new index_fetch() function can only fetch one match from a HOT
chain. That greatly simplifies the code in indexam.c. The only caller
needing to fetch more than one tuple from a HOT chain (= using
SnapshotAny) is CLUSTER, so I moved the HOT-chain following logic into
cluster.c, with small changes to heap_hot_search_buffer().

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com