Export NullAnnotator for v2 parsers

[stephen-soltesz]

This change eliminates dependency on the annotation-service annotator by replacing changing the DefaultAnnotatorFactory to unconditionally return a NullAnnotator instance, now exported from the parser package.

Ultimately, the annotation call outs can be removed from the source entirely.

This change is part of:

• https://github.com/m-lab/etl/issues/1074

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This change is 

[stephen-soltesz]

See for example: https://grafana.mlab-sandbox.measurementlab.net/d/UTgnK-jMz/pipeline-overview?orgId=1&from=now-6h&to=now&var-project=mlab-oti&var-PrometheusDS=Prometheus%20(mlab-oti)&var-LegacyDS=Data%20Proc%20(mlab-oti)&var-Gardener2_DS=Data%20Processing%20(mlab-oti)&var-states=Finishing&var-states=Processing&var-states2=complete&viewPanel=51

[coveralls]

Pull Request Test Coverage Report for Build 7291

• 2 of 12 (16.67%) changed or added relevant lines in 11 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 65.243%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/hopannotation1.go	0	1	0.0%
parser/ndt5_result.go	0	1	0.0%
parser/ndt7_result.go	0	1	0.0%
parser/pcap.go	0	1	0.0%
parser/scamper1.go	0	1	0.0%
parser/ss.go	0	1	0.0%
parser/switch.go	0	1	0.0%
parser/tcpinfo.go	0	1	0.0%
parser/annotation.go	0	2	0.0%
<!--	Total:	2	12	16.67%	-->

Files with Coverage Reduction	New Missed Lines	%
active/active.go	4	88.54%
<!--	Total:	4		-->

Totals
Change from base Build 7286:	-0.07%
Covered Lines:	3972
Relevant Lines:	6088

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💛 - Coveralls