LU-874 ptlrpc: handle in-flight hqreq correctly

LU-874 ptlrpc: handle in-flight hqreq correctly

If there are in-flight requests pending, we shouldn't timeout the
covering dlm locks; neither should we remove the requests from export
exp_hp_rpcs list until the requests are handled.

In this patch, the following things are improved:
1. leave IO rpcs in export's hp list until they are handled;
2. using interval tree to find rpc overlapped locks;
3. refresh the lock again after IO rpcs are finished to leave a time
   window for clients to cancel covering dlm locks;
4. rework repbody in ost_handler.c so as to not modify original obdo
5. cleanup the code.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I22873187aa6fdb28e95173dc0be9f992a54bbb9e
Reviewed-on: http://review.whamcloud.com/2085
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>