Refactor XLogOpenRelation() and XLogReadBuffer() in preparation for relation

Refactor XLogOpenRelation() and XLogReadBuffer() in preparation for relation
forks. XLogOpenRelation() and the associated light-weight relation cache in
xlogutils.c is gone, and XLogReadBuffer() now takes a RelFileNode as argument,
instead of Relation.

For functions that still need a Relation struct during WAL replay, there's a
new function called CreateFakeRelcacheEntry() that returns a fake entry like
XLogOpenRelation() used to.