Removed practice of putting a datasync after every write, we now put a datasync around metablock writes.
Replaced boost::bind with std::bind in disk.cc.
Made metablock_manager used scoped_malloc_t in a place where it should.
Merge branch 'v1.6.x' into sam_520
Removed FILE_SYNC_TECHNIQUE define, since it is only used in disk.cc.
Removed unused header includes from disk.hpp.
Merge branch 'sam_520' into next