Fix pcap packet acquisition methods

Fix pcap packet acquisition methods passing 0 to pcap_dispatch.
Previously they passed the packet pool size, but the packet_q_len
variable was now hardcoded at 0.

This patch sets packet_q_len to 64. If packet pool is empty, we fall
back to direct alloc. As the pcap_dispatch function is only called
when packet pool is not empty, we alloc at most 63 packets.
Fix packet pool pending stack adds

Add packets after the first as the list/stack head as well.
Remove unused variables
Fix engine getting stuck because of optimizations

At -O1+ in both Gcc and Clang, PacketPoolWait would optimize the
wait loop in the wrong way. Adding a compiler barrier to prevent
this optimization issue.