Add MongoDB instrumentation to action logging

We can generate more fine-grained instrumentation of MongoDB queries directly
in the request logs by subscribing to the notifications generated by moped (the
connection adapter used by mongoid). For example:

  Completed 200 OK in 3ms (Views: 1.2ms | Mongo: 0.7ms)
Use DB payload key logstasher is looking for

By using the same payload key as ActiveRecord, we get the DB duration into the
logstasher output for free.

Note that this assumes that the application is only running mongoid and not
ActiveRecord as well, otherwise the recorded duration would get clobbered by one
or the other.
Add integration test for Mongo instrumentation
Merge pull request #70 from alphagov/mongodb-instrumentation

Add MongoDB instrumentation to action logging