Add a new API, getUri(), to all cache instances.

The reason to add this API is:

(1) We added this API to RedisInstance recently for tracking the behavior
    of each individual instance (c9224fc4c4b1b5b5b9b5f1d160486b9c5769b963).
(2) We will need to add this API to CacheCluster so we can query which
    cache server is in charge of a given key.

Given that we have added and will add this API to two types of instances,
and it could be useful in other scenarios as well, so it makes sense to
add to all of them.
Merge pull request #43 from Obvious/xiao-geturi

Add a new API, getUri(), to all cache instances.