L2-LEARN:fix l2fib entry seq num not updated on hit (VPP-888)

L2-LEARN:fix l2fib entry seq num not updated on hit (VPP-888)

fixed instability in l2bd_multi_instnce test - sometimes failing with extra
packets captured

it appears l2-learn was not updating hit entries but rather a copy of them.

if the ager did not have a chance to run before the test was running the
learning cycle - entries were not updated with the packet's seq num - causing
packets to flood when hitting the stale seq_num in l2-fwd - hence the extra
packets

fixed handling of filter entries

revert workaround for instability in test

Change-Id: I16d918e6310a5bf40bad5b7335b2140c2867cb71
Signed-off-by: Eyal Bari <ebari@cisco.com>