examples/l3fwd: fix exact match performance

It seems that for the most use cases, previous hash_multi_lookup provides
better performance, and more, sequential lookup can cause significant
performance drop.

This patch sets previously optional hash_multi_lookup method as default.
It also provides some minor optimizations such as queue drain only on used
tx ports.

Fixes: 94c54b4158d5 ("examples/l3fwd: rework exact-match")
Fixes: dc81ebbacaeb ("lpm: extend IPv4 next hop field")
Fixes: 64d3955de1de ("examples/l3fwd: fix ARM build")

Reported-by: Qian Xu <qian.q.xu@intel.com>
Signed-off-by: Tomasz Kulasek <tomaszx.kulasek@intel.com>