Speed up the DistributedLock test

https://github.com/alphagov/govuk_need_api/pull/68 introduced the DistributedLock
and related tests. This change speeds up the tests by making the lock acquisition timeout
configurable and much shorter (which shaves 9.5 seconds off the unit test run).
Merge pull request #70 from alphagov/speed-up-distributed-lock-test

Speed up the DistributedLock test