Making pcapgo Writer routine safe

[gboddin]

Calling write from parallel routines will cause the pcap file to corrupt.

Using a mutex ensure we don't corrupt the file by delaying concurrent writes.

An alternative method would be to use a buffered channel but requires an additional routine

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[mosajjal]

hey mate, can you move this to gopacket/gopacket and raise it there. I'll merge it over there.