Switch logger to github.com/go-kit/log

Description

I would like to propose a change from klog to github.com/go-kit/log.

The main reasons are:

1. support for logfmt
2. support for contextual logging

Especially contextual logging is something we are dearly missing with klog. This would help a lot when dealing with logging within client/server-specific contexts You can see symptoms of this problem with the number of identical "client", client.ID log fields in for most klog calls in gateway/proxy.go.

It would also be nice to switch to logfmt, because it has far more support by parsers and first class support by Loki. It is intentionally less powerful than klog's format, which means we can't just log anything but have to constrain ourselves to basic types.