Add caching support to Gateway

[trajano]

If an ETag and Expires was provided by the backend service, store it in the gateway's local cache, the cache does not need to be in a Hazelcast as an in-memory cache will suffice.

[trajano]

There would need to be caching buckets based on TTL and requests need to check whether the entry had expired or not when it is expired it will call evict to remove it from the cache.

The scanning has to be done on each bucket.