LU-1030 osc: new IO engine implementation

LU-1030 osc: new IO engine implementation

New IO engine to manage dirty pages with osc_extent.

Osc_extent is a data structure to manage a series of contiguous
blocks; however, the pages in an extent is not required to be
contiguous. An extent must be written out in one RPC.

The purpose of introducing extents are:
1. make grants work for extent-based OSD;
2. form better IO by picking up contiguous pages to compose RPC;
3. reimplement ll_writepages() with CIT_FSYNC.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I3ef619c1f07eefd201236ab55e5fd858791d41e0
Reviewed-on: http://review.whamcloud.com/2270
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>