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