Track the number of URLs we still have to publish

So that we can better understand one of the failure cases (early exit)
we should track the number of URLs that we haven't published back to
the exchange and that we have in progress.

Switched from the Etsy example Go code to another StatsD library
because the original Etsy code didn't support sending gauges.
Use empty struct for signalling channel

Channels that are being used purely for signalling should use an
empty struct rather than boolean or int types.

Using an empty struct declares that we're not interested in the value
sent or received; only in its closed property.

See http://talks.golang.org/2012/10things.slide#11

Follows on from alphagov/external-link-tracker#9
Merge pull request #28 from alphagov/use-empty-struct-for-signally-channel

Use empty struct for signalling channel
Merge pull request #27 from alphagov/track-the-number-of-urls-we-still-need-to-publish

Track the number of URLs we still have to publish
[#73135884] Increase TTL expiry time to 24 hours

Increase the TTL expiry time because the full crawl is currently
taking just over 12 hours. As a result, we end up going round a number
of URLs again after 12 hours have passed.
[#73135884] Log when we write URLs to disk

So that we have some audit history on where we've got to for our
crawled URLs.
Merge pull request #30 from alphagov/log-when-we-write-contents-to-disk

[#73135884] Log when we write URLs to disk
Merge pull request #29 from alphagov/increase-ttl-expiry-to-24-hours

[#73135884] Increase TTL expiry time to 24 hours
[#73135884] Increase back off time for 429 response codes

Increase the amount of time we back off from crawling when we receive
a 429 HTTP status code. Otherwise we won't have waited long enough for
the server to let us continue.

Also remove back off times for 5XX responses as we'll want to count
the number of times we receive a 5XX response and then manage how to
reject the message in the future.
Merge pull request #31 from alphagov/increase-back-off-time-for-429-requests

[#73135884] Increase back off time for 429 response codes
Ensure blacklisted seed URLs not written to disk
.