Port To Jersey 2.x

Note that this relies on a forked branch to Metrics for the same purpose of supporting Jersey2 that hasn't been accepted yet. The PR for that is at https://github.com/dropwizard/metrics/pull/532


I would gladly do that, but the implementation uses a static method org.apache.http.impl.client.HttpClientBuilder.create() to get the org.apache.http.impl.client.HttpClientBuilder instance. One option would be to add a package private setHttpClientBuilder(org.apache.http.impl.client.HttpClientBuilder builder) method just to avoid all this mocking in the testing, but that doesn't sound quite right either as then we're changing the class solely to enable our testing as this is not public behavior. What do you advise?