Pull out common code in trace functions

[gfr10598]

This is not a strict refactoring, but is not intended to change behavior in any way.

Passes unit tests. We should probably also run it in sandbox manually.

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This change is 

[gfr10598]

I've pushed it manually to sandbox ndt pods.

[coveralls]

Pull Request Test Coverage Report for Build 303

• 66 of 68 (97.06%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.9%) to 97.97%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tracer/scamper.go	66	68	97.06%
<!--	Total:	66	68	97.06%	-->

Files with Coverage Reduction	New Missed Lines	%
tracer/scamper.go	4	93.51%
<!--	Total:	4		-->

Totals
Change from base Build 301:	-0.9%
Covered Lines:	579
Relevant Lines:	591

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💛 - Coveralls

[SaiedKazemi]

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tracer/scamper.go, line 83 at r1 (raw file):

// called binaries have a much smaller "blast radius".
func (s *Scamper) Trace(conn connection.Connection, t time.Time) (out []byte, err error) {
  // XXX Is this still useful?  Does shx.PipeJob we ever panic?

shx does not panic, so this recover() should never happen. But I suggest we change (correct) error/panic/recovery handling in a separate PR.

[SaiedKazemi]

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tracer/scamper.go, line 180 at r1 (raw file):

// connection.
func (d *ScamperDaemon) Trace(conn connection.Connection, t time.Time) (out []byte, err error) {
  // XXX Is this still useful?  Does shx.PipeJob we ever panic?

See my previous comment.

[SaiedKazemi]

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tracer/scamper.go, line 272 at r1 (raw file):

      }
      return buff.Bytes(), err
  } else {

Please remove this unnecessary else.