Fix circular import

[StoneDot]

This PR fixes the circular import problems occurring when you load http2 or https module before http module.

For example, the following code reproduces a problem @opencensus-instrumentation-http module doesn't load properly.

const tracing = require('@opencensus/nodejs');
const zipkin = require('@opencensus/exporter-zipkin');
const assert = require('assert');

const tracer = tracing.start({samplingRate: 1.0}).tracer;

const https = require('https');
const http = require('http');

tracer.registerSpanEventListener(new zipkin.ZipkinTraceExporter({
    url: 'http://localhost:9411/api/v2/spans',
    serviceName: 'node.js-open-census-bug'
}));

http.get('http://example.com/', (res) => {
  res.setEncoding('utf8');
  let rawData = '';
  res.on('data', (chunk) => { rawData += chunk; });
  res.on('end', () => {
    console.log(rawData);
  });
});

The process of problem is as follows schematically:

1. Load http2 module.
2. require-in-the-middle load @opencensus-instrumentation-http2 module.
3. @opencensus-instrumentation-http2 load @opencensus-instrumentation-http module.
4. @opencensus-instrumentation-http load http.
5. require-in-the-middle try to load @opencensus-instrumentation-http, but loading of @opencensus-instrumentation-http is already in progress. Therefore require statement return partial @opencensus-instrumentation-http.
6. The problem happens.

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[StoneDot]

@googlebot I signed it!

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[mayurkale22]

@draffensperger and @hekike what do you think about this?

[draffensperger]

LGTM, though I think it's important to comment why this is needed.

[draffensperger]

Could you add a comment for why this line is needed to avoid the circular import? And similar for the other case below?