Add redis pool to registry webapp

Redis has been integrated with the web application for use with various
services. The configuraiton exposes connection details, timeouts and pool
parameters. Documentation has been updated accordingly.

A few convenience methods have been added to the context package to get loggers
with certain fields, exposing some missing functionality from logrus.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/registry/config.yml

@@ -12,6 +12,15 @@ http:
     secret: asecretforlocaldevelopment
     debug:
         addr: localhost:5001
+redis:
+  addr: localhost:6379
+  pool:
+    maxidle: 16
+    maxactive: 64
+    idletimeout: 300s
+  dialtimeout: 10ms
+  readtimeout: 10ms
+  writetimeout: 10ms
 notifications:
     endpoints:
         - name: local-8082