lib/classifier: Add subtable cache diagnostics.

Assert failures that should not happen have been reported on some
(non-OVS) test cases.  This patch adds diagnostics to analyze what
goes wrong.

None of the OVS unit tests trigger these, so there is no performance
penalty.

This could be moved to test-classifier once it has served it's
purpose.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>