Add a persistent connection for librato

When sending metrics and annotations to librato, use a persistent HTTP
connection.  clj-http.conn-mgr/make-reusable-conn-manager options can be passed
as a final map argument to librato-metrics to control the number of threads
used (default 4).
Use update-annotation
riemann.time.controlled: Use of with-redefs to be slightly more forgiving.
Merge pull request #297 from udoprog/udoprog/with-redefs

riemann.time.controlled: Use of with-redefs to be slightly more forgiving.
Merge pull request #285 from hugoduncan/feature/add-librato-persistent-connection

Add a persistent connection for librato