basic_apps: cache link.{transmit,receive}
don't ipairs deep in loop
transmit: avoid %
free: don't fill-0 the packet.    XXX: safe?
comment out deep assert()ions
Revert "free: don't fill-0 the packet.    XXX: safe?"

This reverts commit d696a405c3c988256f69bb0ac9ff48a08ab74c02.
Merge pull request #203 from llelf/hurry1

some speed ups (based on bench/basic1)