IMPALA-122: Fix FindFirstBlock logic.

Change-Id: I34e5ce2f6c112fdbcdd290faeff7d044ed77127d
Fix IMPALA-129, IMPALA-534, and other scanner bugs.

Change-Id: I7906f291fe6fd1bcfc96b8b03ed42ff31562b793
IMPALA-525: Sequence scanners use average block size to minimize reads past eosr

Change-Id: I33c198a5958ca8110f67682f7d51cfaf55b0bbd6