Switch Logging To Zerolog

[d80tb7]

• Define a New logging Abstraction in common/logging to allow creattion of Loggers or gloabl logging (e.g. log.Info()). This means that none of our code outside of the logging package depends on any logging implementation.
• Define a zerolog implementation of the logging abstraction.
• Updates the code to use the new logging abstraction.
• Implements a prometheus hook which recreates https://github.com/weaveworks/promrus so that we get prometheus metrics about log levels. The metric name hasn't changed so this should be backwards compatible.