work around IPv6 addresses in Trace#fromRequest

[flugeldoo]

When tracing systems on IPv6 networks, transmission to the collector fails.

error:  Error: IPv4 string does not have 4 parts.
    at ipv4ToNumber (.../node_modules/tryfer/lib/formatters.js:123:13)
    at .../node_modules/tryfer/lib/formatters.js:157:17
    at Array.forEach (native)
    at Object.formatForZipkin (.../node_modules/tryfer/lib/formatters.js:153:29)
    at RawZipkinTracer._sendTrace (.../node_modules/tryfer/lib/node_tracers.js:219:14)
    at replenish (.../node_modules/tryfer/node_modules/async/lib/async.js:144:17)
    at Object.async.forEachLimit (.../node_modules/tryfer/node_modules/async/lib/async.js:161:11)
    at RawZipkinTracer.sendTraces (.../node_modules/tryfer/lib/node_tracers.js:230:9)
    at BufferingTracer._sendTraces (.../node_modules/tryfer/lib/node_tracers.js:100:16)
    at BufferingTracer._periodicSendFunction (.../node_modules/tryfer/lib/node_tracers.js:90:10)
    at Timer.listOnTimeout (timers.js:119:15)

Zipkin's Thrift schema doesn't accommodate IPv6 addresses, so the only way to deal with them is to mask them.

[suryatech]

@niccaluim Can you please fix the lint errors that are outlined here?

I am not sure if we should write our own IPv4 validator. Have you considered using a regular expression for validation purposes?

Everything apart from this looks good to me.

[suryatech]

Here's a small perf test