Define a separately configurable XLOG_BLCKSZ symbol for the page size

Define a separately configurable XLOG_BLCKSZ symbol for the page size
used within WAL files.  Historically this was the same as the data file
BLCKSZ, but there's no necessary connection, and it's possible that
performance gains might ensue from reducing XLOG_BLCKSZ.  In any case
distinguishing two symbols should improve code clarity.  This commit
does not actually change the page size, only provide the infrastructure
to make it possible to do so.  initdb forced because of addition of a
field to pg_control.
Mark Wong, with some help from Simon Riggs and Tom Lane.