Tx feature node, for accounting purposes

Switch to combined allow/drop counters
Show matching ip4 neighbor address if known
Add static-allow mactime entries for unknown mac addresses
Add the "clear mactime" command

Change-Id: Ib963981438dfb8a123df1b3c023bd5fcc27f888f
Signed-off-by: Dave Barach <dbarach@cisco.com>