Introduce compact WAL record for the common case of commit (non-DDL).

Introduce compact WAL record for the common case of commit (non-DDL).
XLOG_XACT_COMMIT_COMPACT leaves out invalidation messages and relfilenodes,
saving considerable space for the vast majority of transaction commits.
XLOG_XACT_COMMIT keeps same definition as XLOG_PAGE_MAGIC 0xD067 and earlier.

Leonardo Francalanci and Simon Riggs