As seen in #1594 – Linkerd occasionally throws `ReaderDiscarded` exceptions when discarding a response on retries. Since nothing is actually wrong, we want to silence these exceptions.

I've added a `Try` to the call to `response.discard` in `RetryFilter.scala`. The `IOException` "Connection reset by peer" is not silenced, since it appears to be both thrown and caught in code we don't own & it would be difficult to quash this exception only on retries.

Tested this against the Kubernetes config described by @esbie in #1594 (comment); the `RetryDiscarded` exception no longer appears in the logs. 

Closes #1594.

## 1.2.0 2017-09-07

* **Breaking Change**: `io.l5d.mesh`, `io.l5d.thriftNameInterpreter`, linkerd
  admin, and namerd admin now serve on 127.0.0.1 by default (instead of
  0.0.0.0).
* **Breaking Change**: Removed support for PKCS#1-formatted keys. PKCS#1 formatted keys must be converted to PKCS#8 format.
* Added experimental `io.l5d.dnssrv` namer for DNS SRV records (#1611)
* Kubernetes
  * Added an experimental `io.l5d.k8s.configMap` interpreter for reading dtabs from a Kubernetes ConfigMap (#1603). This interpreter will respond to changes in the ConfigMap, allowing for dynamic dtab updates without the need to run Namerd.
  * Made ingress controller's ingress class annotation configurable (#1584).
  * Fixed an issue where Linkerd would continue routing traffic to endpoints of a service after that service was removed (#1622).
  * Major refactoring and performance improvements to `io.l5d.k8s` and `io.l5d.k8s.ns` namers (#1603).
  * Ingress controller now checks all available ingress resources before using a default backend (#1607).
  * Ingress controller now correctly routes requests with host headers that contain ports (#1607).
* HTTP/2
  * Fixed an issue where long-running H2 streams would eventually hang (#1598).
  * Fixed a memory leak on long-running H2 streams (#1598)
  * Added a user-friendly error message when a HTTP/2 router receives a HTTP/1 request (#1618)
* HTTP/1
  * Removed spurious `ReaderDiscarded` exception logged on HTTP/1 retries (#1609)
* Consul
  * Added support for querying Consul by specific service health states (#1601)
  * Consul namers and Dtab store now fall back to a last known good state on Consul observation errors (#1597)
  * Improved log messages for Consul observation errors (#1597)
* TLS
  * Removed support for PKCS#1 keys (#1590)
  * Added validation to prevent incompatible `disableValidation: true` and `clientAuth` settings in TLS client configurations (#1621)
* Changed `io.l5d.mesh`, `io.l5d.thriftNameInterpreter`, linkerd
  admin, and namerd admin to serve on 127.0.0.1 by default (instead of
  0.0.0.0) (#1366)
* Deprecated `io.l5d.statsd` telemeter.