Tweak slice allocation to improve pcap parsing performance.

[gfr10598]

This is a minor tweak, but useful to get a better baseline before adding a lot more code.

It also adds a better benchmark, that includes accounting for the packets/op, so that it is easier to estimate time and memory allocated per packet.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 6948

• 14 of 18 (77.78%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 64.245%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/pcap.go	14	18	77.78%

Totals
Change from base Build 6945:	0.03%
Covered Lines:	3856
Relevant Lines:	6002

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💛 - Coveralls

[cristinaleonr]

Reviewable status: 0 of 1 approvals obtained (waiting on @gfr10598)

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parser/pcap.go, line 80 at r2 (raw file):

	pcapSize := len(data) // Only if the data is not compressed.
	// Check magic number?
	if len(data) < 4 {

Would it be more helpful to check for the truncated PCAP earlier?

[stephen-soltesz]

Closing old PR - and referencing the notes from this change in #1085