Implement graceful shutdown and propagate request context (#468)

* feature: Implement graceful shutdown

Propagate the request context to the Redis client.
It is possible to propagate a context cancel to Redis client if the connection is closed by the HTTP client.
The redis.Cmdable cannot use WithContext, so added the Client interface to handle redis.Client and redis.ClusterClient transparently.

Added handling of Unix signals to http server.

Upgrade go-redis/redis to v7.

* Update dependencies

- Upgrade golang/x/* and google-api-go
- Migrate fsnotify import from gopkg.in to github.com
- Replace bmizerany/assert with stretchr/testify/assert

* add doc for  wrapper interface

* Update CHANGELOG.md

* fix: upgrade fsnotify to v1.4.9

* fix: remove unnessary logging

* fix: wait until  all connections have been closed

* refactor: move chan to main for testing

* add assert to check if stop chan is empty

* add an idiomatic for sync.WaitGroup with timeout