Merge Google's Scrubing feature:

commit c01fc2134d439282a21a2ddf687566e198ddee28
Author: Jonas Oreland <jonaso@google.com>
Date:   Thu Apr 10 15:04:25 2014 +0000

    innodb datafile scrubbing

    this patch implements scrubbing of innodb datafiles
    either by scrubbing as part of purge (non compressed)
    or scrubbing by scanning whole tablespaces (added into
    key rotation threads).

    the background scrubbing is further divided into
    btr_optimistic_scrub (which calls btr_page_reorganize) or if that fails
    btr_pessimistic_scrub (which calls page split routines).

    Change-Id: I55915e9274ab3b0c5ac90b42d7304386124b0b0f

commit aad1e7970951220a8869a7e859db574cb4d47649
Author: Minli Zhu <minliz@google.com>
Date:   Mon Feb 10 15:42:26 2014 -0800

    Innodb redo log scrubbing.

    Change-Id: I0d4a55b0021dde7129f6ff701c7b75c4186f9e9a