Switch to the nginx txid extension for providing unique request ids.

The eventual plan is to use this as the actual record ID for populating
elasticsearch. This ID is more space efficient than what we're currently
using, and should theoretically help indexing performance (since it's
sorted). Probably a premature optimization, but seems simple enough.
See:

http://www.elasticsearch.org/blog/performance-considerations-elasticsearch-indexing/
http://blog.mikemccandless.com/2014/05/choosing-fast-unique-identifier-uuid.html