Add unit test for flow condition.

Other changes:

  * Increase UT coverage.
  * API javadoc: Add notice of limitation on flow filter.
  * IcmpMatchImpl: Refine hashCode() for better performance.
  * InetMatchImpl: Ditto.
  * Fix typo in comment.
  * Trivial improvement of error message.

Change-Id: I4f80cc944dba0b841e83615d6ca1e4d4db12a7d5
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>