Rename XLOG_BTREE_NEWPAGE xlog record type into XLOG_HEAP_NEWPAGE, and

Rename XLOG_BTREE_NEWPAGE xlog record type into XLOG_HEAP_NEWPAGE, and
shift support code into heapam.c accordingly.  This is in service of
soon-to-be-committed ALTER TABLE SET TABLESPACE code that will want to
use this same record type for both heaps and indexes.

Theoretically I should have forced initdb for this, but in practice there
is no change in xlog contents because CVS tip will never really emit this
record type anyhow...