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
Test for (where (... event ...))
Merge branch 'master' of github.com:aphyr/riemann
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
streams/scale: docstring example
Merge branch 'master' of github.com:aphyr/riemann
coalesce splits by host as well
Merge pull request #302 from tcrayford/coalesce_docs

coalesce splits by host as well