Bug 2369: Fixed NPE in update-flow RPC.

  - Need to check whether match in update-flow input is null.
  - Interpret null priority in update-flow input as default priority.
  - Original flow should be removed if any of the following fields is
    changed.
    - idle-timeout
    - hard-timeout
    - flags
    - cookie

(cherry picked from commit 021542f27de529f3a253569b34981090a684a26a)
Change-Id: I64c4ff66ce4965cbd8c9e9a608082ef2b5a84439
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>