cluster connection proxy for multi-cluster solutions

FDB multi-cluster solutions need a client that coordinate between multiple clusters and cluster connection interfaces. This change is required by [SnowCannon](https://github.com/apple/foundationdb/issues/1272), and can be used by DR solutions as well.

This change involves two pieces:

Allow cluster connection interface (CCI) to be saved in an FDB database as in a key-value pair

A watcher actor that watches these CCI(s) on the DB and 'switches' the client to the new interface when a change is triggered. The trigger must also invalidate cached key locations and reset watches through the new CCI.