pcap: rewrite getNextBufPtrLocked loop
pcap: rewrite getNextBufPtrLocked loop

Rewrite `getNextBufPtrLocked` to try to read a packet first before
calling `pcap_wait` which uses an expensive `select` syscall. Replaced
`stop` channel with atomic uint64.

google/gopacket,google/gopacket,google/gopacket