Fix bug in gplibpq.c for handling CONTRECORD.
Fix bug in gplibpq.c for handling CONTRECORD.

Originally, we compute the offset of CONTRECORD, and compose the full
xlaorecord, and then just extract the header information. During the
composition of the xlaorecord, we point to a wrong location for the
continous record, which results in incorrect parsing of XLOG records.

Now, we only extract the header, and move offset to the proper page
boundary, so that the common page header code can handle the
CONTRECORD.

Author: Xin Zhang <80ac6425c128040c27b3c6077c824bc26222f5e7@pivotal.io>
Author: Ashwin Agrawal <924bf3437ad315c1c74f9c41d83153600b05f548@pivotal.io>

adam8157/gpdb,edespino/gpdb,50wu/gpdb,Chibin/gpdb,xinzweb/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,lisakowen/gpdb,ashwinstar/gpdb,edespino/gpdb,50wu/gpdb,yuanzhao/gpdb,lisakowen/gpdb,Chibin/gpdb,50wu/gpdb,50wu/gpdb,Chibin/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,jmcatamney/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,yuanzhao/gpdb,Chibin/gpdb,adam8157/gpdb,Chibin/gpdb,xinzweb/gpdb,edespino/gpdb,greenplum-db/gpdb,edespino/gpdb,edespino/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,lisakowen/gpdb,Chibin/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,Chibin/gpdb,ashwinstar/gpdb,lisakowen/gpdb,50wu/gpdb,yuanzhao/gpdb,adam8157/gpdb,adam8157/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,edespino/gpdb,Chibin/gpdb,edespino/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,yuanzhao/gpdb,50wu/gpdb,xinzweb/gpdb,Chibin/gpdb,edespino/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,Chibin/gpdb,greenplum-db/gpdb,xinzweb/gpdb,yuanzhao/gpdb,edespino/gpdb,edespino/gpdb,yuanzhao/gpdb,xinzweb/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb