IMPALA-1401: raise MAX_PAGE_HEADER_SIZE and use scanner context to
stitch together header buffer

Change-Id: I4f33b90e845e9bef1ac929bf4ebb8e98eaff985c
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4961
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins
(cherry picked from commit c3a90183b2f03434a9604f3aa2ef6dd08c9ba97c)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4981
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>