LU-2240 fld: Move range lookup to fld.

Because ZFS does not support range lookup in a ZAP, we will move
range lookup  to FLD, and do an in-memory lookup there.  The FLD
entries are still stored in an index on disk, but this is just a
container and is not used for searching anymore.

It is expected the FLD will always be able to fit in RAM.

Signed-off-by: Wang Di <di.wang@whamcloud.com>
Change-Id: If8cadcc30743e6d63ba779a430bc585af29d51fe
Reviewed-on: http://review.whamcloud.com/4396
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>